diff --git a/README.md b/README.md
index 7060966..a862563 100644
--- a/README.md
+++ b/README.md
@@ -210,9 +210,11 @@ effective settings are logged at start.
   `https`, a host and an optional port, and nothing more.
 - `SWWAF_INSTANCE_NAME` (default: the host's name, which docker sets to the
   first 12 characters of the container's id unless the deployment names one):
-  the name each request log line gives as `instance`. Set it, for example to
+  the name every log line and alert gives as `instance`, and every metric
+  carries as its label `instance` (see "Metrics" below). Set it, for example to
   `fsn1app1/gitea`, for a name that stays the same when a deploy replaces the
-  container, and that tells instances apart when several log to one place.
+  container, and that tells instances apart when several log to one place. A
+  name that is not valid UTF-8, such as one saved in Latin-1, stops the start.
 - `SWWAF_MODE` (default `enforce`): `enforce`, or `observe` to pass on the
   requests `smallwebwaf` would refuse and log what it would have done (see "What
   it does so far" above).
@@ -513,7 +515,7 @@ A field that does not apply to a request is left out of its line, apart from
 
 No body is logged, and no header but those above. `smallwebwaf`'s own messages
 (start, the settings, stop, errors) share the stream as JSON lines marked
-`"type":"process"`.
+`"type":"process"`, each with `instance` as a request's line has it.
 
 Go's HTTP server, on which `smallwebwaf` is built, reads a request's line and
 headers before `smallwebwaf` sees the request, and some requests end there,
@@ -897,6 +899,13 @@ empty directory or set `SWWAF_RULES_ENABLED=false`.
 format, for a scraper that sends `SWWAF_METRICS_TOKEN`, through traefik like any
 other request. No metric carries a client's address.
 
+Every metric below, Go's and the process's included, carries the label
+`instance`, `SWWAF_INSTANCE_NAME`, as a constant label set once on the registry
+the metrics are kept in, rather than as a label each metric declares. Prometheus
+gives each series it scrapes an `instance` label of its own, the address it
+scraped, and keeps this one as `exported_instance` unless the scrape sets
+`honor_labels: true`.
+
 - `smallwebwaf_requests_total`, `smallwebwaf_request_bytes_total` and
   `smallwebwaf_response_bytes_total`: requests, and their body bytes each way,
   by `status_class`, such as `2xx`, or `none` when nothing was sent, and by
diff --git a/internal/config/config.go b/internal/config/config.go
index 1f4e59e..dd759fb 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -34,9 +34,10 @@ type Config struct {
 	ListenAddr string
 	// UpstreamURL is the app (SWWAF_UPSTREAM_URL).
 	UpstreamURL *url.URL
-	// InstanceName is the name each request log line gives as instance
-	// (SWWAF_INSTANCE_NAME), by default the host's name, which docker sets
-	// to the first 12 characters of the container's id.
+	// InstanceName is the name every log line and alert gives as instance,
+	// and every metric carries as its label instance (SWWAF_INSTANCE_NAME),
+	// by default the host's name, which docker sets to the first 12
+	// characters of the container's id.
 	InstanceName string
 	// Observe is true in observe mode, when SWWAF_MODE is observe rather
 	// than enforce: a request that SWWAF_DENY_NETS, a ban, the country
@@ -267,6 +268,7 @@ var (
 		"is not ban, permanent_ban, waf_block, anomaly, reputation_hit, " +
 			"source_failure or file_error")
 	errNotNumberOrOff = errors.New("is not a whole number above zero, such as 60, or off")
+	errNotUTF8        = errors.New("is not valid UTF-8")
 )
 
 // FromEnvironment reads the settings with lookupEnv, normally
@@ -276,11 +278,10 @@ var (
 // setting that is set but invalid is an error that names it.
 func FromEnvironment(lookupEnv func(string) (string, bool)) (*Config, error) {
 	env := &environment{lookupEnv: lookupEnv}
-	hostname, _ := os.Hostname() // "" when the host has no name to give
 	cfg := &Config{
 		ListenAddr:           env.address("SWWAF_LISTEN_ADDR", defaultListenAddr),
 		UpstreamURL:          env.appURL("SWWAF_UPSTREAM_URL", defaultUpstreamURL),
-		InstanceName:         env.value("SWWAF_INSTANCE_NAME", hostname),
+		InstanceName:         env.instanceName(),
 		Observe:              env.observe("SWWAF_MODE", "enforce"),
 		TrustedProxies:       env.netblocks("SWWAF_TRUSTED_PROXIES", privateRanges),
 		ClientRequestTimeout: env.duration("SWWAF_CLIENT_REQUEST_TIMEOUT", "60s"),
@@ -373,6 +374,16 @@ func ListenAddrAndUpstreamURL(
 	return listenAddr, upstreamURL, nil
 }
 
+// InstanceName reads only SWWAF_INSTANCE_NAME, which may be given as a
+// file, as FromEnvironment does, so that the line saying a setting is
+// invalid carries it too. A file that cannot be read gives the default
+// here, and FromEnvironment then stops the start over it.
+func InstanceName(lookupEnv func(string) (string, bool)) string {
+	env := &environment{lookupEnv: lookupEnv}
+
+	return env.instanceName()
+}
+
 // privateRanges are the private address ranges, the default trusted
 // proxies.
 const privateRanges = "10.0.0.0/8,172.16.0.0/12,192.168.0.0/16"
@@ -660,6 +671,19 @@ func (e *environment) facility(name, defaultValue string) int {
 	return number
 }
 
+// instanceName reads SWWAF_INSTANCE_NAME, by default the host's name. It
+// must be valid UTF-8: the metrics library panics on a label that is not.
+func (e *environment) instanceName() string {
+	hostname, _ := os.Hostname() // "" when the host has no name to give
+
+	value := e.value("SWWAF_INSTANCE_NAME", hostname)
+	if !utf8.ValidString(value) {
+		e.check("SWWAF_INSTANCE_NAME", fmt.Errorf("%q %w", value, errNotUTF8))
+	}
+
+	return value
+}
+
 // appName reads the setting that is the APP-NAME of the records the log
 // lines are sent in, by default the instance name. Its value is checked
 // when it is set, and, while lines are sent, when it is the instance name.
diff --git a/internal/config/config_test.go b/internal/config/config_test.go
index f1df225..ecd53fa 100644
--- a/internal/config/config_test.go
+++ b/internal/config/config_test.go
@@ -737,6 +737,25 @@ func TestInstanceNameWithAControlCharacterStopsTheStartOnlyWithNtfySet(t *testin
 	}
 }
 
+func TestInstanceNameNotUTF8StopsTheStart(t *testing.T) {
+	t.Parallel()
+
+	// café saved in Latin-1.
+	const latin1 = "caf\xe9"
+
+	for name, env := range map[string]environment{
+		"set":       {instanceName: latin1},
+		"in a file": {instanceName + "_FILE": writeFile(t, latin1+"\n")},
+	} {
+		_, err := config.FromEnvironment(env.lookupEnv)
+
+		want := instanceName + `: "caf\xe9" is not valid UTF-8`
+		if err == nil || err.Error() != want {
+			t.Errorf("%s: error %v, want %s", name, err, want)
+		}
+	}
+}
+
 func TestCodeOnBothCountryListsStopsTheStart(t *testing.T) {
 	t.Parallel()
 
diff --git a/internal/lookup/lookup_test.go b/internal/lookup/lookup_test.go
index 8f24cc6..1f85297 100644
--- a/internal/lookup/lookup_test.go
+++ b/internal/lookup/lookup_test.go
@@ -199,7 +199,7 @@ func TestFailureIsLoggedWithoutTheAddressesAskedAbout(t *testing.T) {
 			URL:        lookup.URL,
 			Now:        time.Now,
 			ProcessLog: slog.New(slog.NewTextHandler(&log, nil)),
-			Metrics:    metrics.New(1),
+			Metrics:    metrics.New(1, "app"),
 			Alerts:     alerts.New(alerts.Params{}),
 		})
 		g.SetTransport(geojs)
@@ -398,7 +398,7 @@ func TestClientsWithoutAnAnswerAreCounted(t *testing.T) {
 	t.Parallel()
 
 	synctest.Test(t, func(t *testing.T) {
-		m := metrics.New(1)
+		m := metrics.New(1, "app")
 		g := lookup.New(lookup.Params{
 			URL:        lookup.URL,
 			Now:        time.Now,
@@ -589,7 +589,7 @@ func startWithAlerts() (*standIn, *testClock, *lookup.GeoJS, *alerts.Queue) {
 		URL:        lookup.URL,
 		Now:        clock.Now,
 		ProcessLog: slog.New(slog.DiscardHandler),
-		Metrics:    metrics.New(1),
+		Metrics:    metrics.New(1, "app"),
 		Alerts:     queue,
 	})
 	g.SetTransport(geojs)
diff --git a/internal/metrics/metrics.go b/internal/metrics/metrics.go
index 410f1f2..3eb8df2 100644
--- a/internal/metrics/metrics.go
+++ b/internal/metrics/metrics.go
@@ -21,7 +21,8 @@ import (
 
 // Metrics are smallwebwaf's metrics. They are safe for concurrent use.
 type Metrics struct {
-	registry *prometheus.Registry
+	// registry gives every metric registered with it the label instance.
+	registry prometheus.Registerer
 	handler  http.Handler
 
 	inFlight             prometheus.Gauge
@@ -55,13 +56,17 @@ type Metrics struct {
 
 // New returns the metrics, with the Go runtime's and the process's own.
 // topN is how many countries get series of their own
-// (SWWAF_METRICS_TOP_N).
-func New(topN int) *Metrics {
+// (SWWAF_METRICS_TOP_N). Every metric carries instanceName
+// (SWWAF_INSTANCE_NAME) as its label instance.
+func New(topN int, instanceName string) *Metrics {
 	byStatus := []string{"status_class", "action"}
 	byFile := []string{"file"}
+	registry := prometheus.NewRegistry()
 
 	m := &Metrics{
-		registry: prometheus.NewRegistry(),
+		registry: prometheus.WrapRegistererWith(
+			prometheus.Labels{"instance": instanceName}, registry),
+		handler: promhttp.HandlerFor(registry, promhttp.HandlerOpts{}),
 		inFlight: prometheus.NewGauge(prometheus.GaugeOpts{
 			Name: "smallwebwaf_requests_in_flight",
 			Help: "Requests under way.",
@@ -119,8 +124,6 @@ func New(topN int) *Metrics {
 			byFile),
 	}
 
-	m.handler = promhttp.HandlerFor(m.registry, promhttp.HandlerOpts{})
-
 	m.registry.MustRegister(
 		collectors.NewGoCollector(),
 		collectors.NewProcessCollector(collectors.ProcessCollectorOpts{}),
diff --git a/internal/proxy/metrics_test.go b/internal/proxy/metrics_test.go
index 1d82beb..a6c3881 100644
--- a/internal/proxy/metrics_test.go
+++ b/internal/proxy/metrics_test.go
@@ -148,8 +148,8 @@ func TestMetricsCountTheTraffic(t *testing.T) {
 	wantStatus(t, get(t, addr, "/_smallwebwaf/nothing"), http.StatusNotFound)
 	out.requestLines(t, 2)
 
-	forward := `{action="forward",status_class="2xx"}`
-	notFound := `{action="admin",status_class="4xx"}`
+	forward := `{action="forward",instance="app",status_class="2xx"}`
+	notFound := `{action="admin",instance="app",status_class="4xx"}`
 
 	// The request for the metrics is itself under way.
 	metrics := scrape(t, addr)
@@ -159,11 +159,13 @@ func TestMetricsCountTheTraffic(t *testing.T) {
 	wantMetric(t, metrics, "smallwebwaf_response_bytes_total"+forward, 5)
 	wantMetric(t, metrics, "smallwebwaf_response_bytes_total"+notFound,
 		float64(len("Not Found\n")))
-	wantMetric(t, metrics, "smallwebwaf_request_duration_seconds_count", 2)
-	wantMetric(t, metrics, "smallwebwaf_upstream_duration_seconds_count", 1)
-	wantMetric(t, metrics, "smallwebwaf_requests_in_flight", 1)
-	metric(t, metrics, "go_goroutines")
-	metric(t, metrics, "process_start_time_seconds")
+	wantMetric(t, metrics,
+		`smallwebwaf_request_duration_seconds_count{instance="app"}`, 2)
+	wantMetric(t, metrics,
+		`smallwebwaf_upstream_duration_seconds_count{instance="app"}`, 1)
+	wantMetric(t, metrics, `smallwebwaf_requests_in_flight{instance="app"}`, 1)
+	metric(t, metrics, `go_goroutines{instance="app"}`)
+	metric(t, metrics, `process_start_time_seconds{instance="app"}`)
 
 	// A request the app holds is under way until it ends.
 	httpClient := newClient(t)
@@ -180,7 +182,7 @@ func TestMetricsCountTheTraffic(t *testing.T) {
 	}()
 
 	<-arrived
-	wantMetric(t, scrape(t, addr), "smallwebwaf_requests_in_flight", 2)
+	wantMetric(t, scrape(t, addr), `smallwebwaf_requests_in_flight{instance="app"}`, 2)
 	releaseApp()
 
 	err := <-ended
@@ -189,7 +191,7 @@ func TestMetricsCountTheTraffic(t *testing.T) {
 	}
 
 	out.requestLines(t, 5)
-	wantMetric(t, scrape(t, addr), "smallwebwaf_requests_in_flight", 1)
+	wantMetric(t, scrape(t, addr), `smallwebwaf_requests_in_flight{instance="app"}`, 1)
 }
 
 func TestMetricsCountLimitsAndBans(t *testing.T) {
@@ -219,15 +221,16 @@ func TestMetricsCountLimitsAndBans(t *testing.T) {
 
 	metrics := s.scrape(scraper)
 	wantMetric(t, metrics,
-		`smallwebwaf_requests_total{action="denied",status_class="none"}`, 1)
-	wantMetric(t, metrics, `smallwebwaf_rate_limit_hits_total{window="minute"}`, 1)
-	wantMetric(t, metrics, `smallwebwaf_offences_total{kind="limit"}`, 1)
-	wantMetric(t, metrics, `smallwebwaf_bans_made_total{cause="limit"}`, 1)
-	wantMetric(t, metrics, "smallwebwaf_active_bans", 1)
-	wantMetric(t, metrics, "smallwebwaf_permanent_bans", 0)
+		`smallwebwaf_requests_total{action="denied",instance="app",status_class="none"}`, 1)
+	wantMetric(t, metrics,
+		`smallwebwaf_rate_limit_hits_total{instance="app",window="minute"}`, 1)
+	wantMetric(t, metrics, `smallwebwaf_offences_total{instance="app",kind="limit"}`, 1)
+	wantMetric(t, metrics, `smallwebwaf_bans_made_total{cause="limit",instance="app"}`, 1)
+	wantMetric(t, metrics, `smallwebwaf_active_bans{instance="app"}`, 1)
+	wantMetric(t, metrics, `smallwebwaf_permanent_bans{instance="app"}`, 0)
 
 	clk.advance(time.Hour)
-	wantMetric(t, s.scrape(scraper), "smallwebwaf_active_bans", 0)
+	wantMetric(t, s.scrape(scraper), `smallwebwaf_active_bans{instance="app"}`, 0)
 
 	// A limit broken again right after would ban for three hours, longer
 	// than SWWAF_MAX_BAN_DURATION, so the ban is permanent.
@@ -235,13 +238,14 @@ func TestMetricsCountLimitsAndBans(t *testing.T) {
 	s.get(client, 0, requestlog.ActionRateLimited)
 
 	metrics = s.scrape(scraper)
-	wantMetric(t, metrics, `smallwebwaf_rate_limit_hits_total{window="minute"}`, 2)
-	wantMetric(t, metrics, `smallwebwaf_offences_total{kind="limit"}`, 2)
-	wantMetric(t, metrics, `smallwebwaf_bans_made_total{cause="limit"}`, 2)
-	wantMetric(t, metrics, "smallwebwaf_active_bans", 1)
-	wantMetric(t, metrics, "smallwebwaf_permanent_bans", 1)
+	wantMetric(t, metrics,
+		`smallwebwaf_rate_limit_hits_total{instance="app",window="minute"}`, 2)
+	wantMetric(t, metrics, `smallwebwaf_offences_total{instance="app",kind="limit"}`, 2)
+	wantMetric(t, metrics, `smallwebwaf_bans_made_total{cause="limit",instance="app"}`, 2)
+	wantMetric(t, metrics, `smallwebwaf_active_bans{instance="app"}`, 1)
+	wantMetric(t, metrics, `smallwebwaf_permanent_bans{instance="app"}`, 1)
 	// denied, client, and the scraper as of its earlier requests.
-	wantMetric(t, metrics, "smallwebwaf_tracked_clients", 3)
+	wantMetric(t, metrics, `smallwebwaf_tracked_clients{instance="app"}`, 3)
 }
 
 func TestMetricsCountTheBansAnAdminMakes(t *testing.T) {
@@ -255,7 +259,7 @@ func TestMetricsCountTheBansAnAdminMakes(t *testing.T) {
 		rateLimitExemptNets: scraper,
 	})
 
-	const admins = `smallwebwaf_bans_made_total{cause="admin"}`
+	const admins = `smallwebwaf_bans_made_total{cause="admin",instance="app"}`
 
 	wantMetric(t, s.scrape(scraper), admins, 0)
 
@@ -319,28 +323,42 @@ func TestMetricsByCountryKeepTheBusiestAndCountTheRestAsOther(t *testing.T) {
 	metrics := scrape(t, addr)
 	lines++
 
-	wantMetric(t, metrics, `smallwebwaf_country_requests_total{country="KP"}`, 3)
-	wantMetric(t, metrics, `smallwebwaf_country_requests_total{country="DE"}`, 2)
-	wantMetric(t, metrics, `smallwebwaf_country_requests_total{country="other"}`, 1)
-	wantMetric(t, metrics, `smallwebwaf_country_list_refusals_total{country="KP"}`, 3)
-	wantMetric(t, metrics, `smallwebwaf_country_request_bytes_total{country="KP"}`, 0)
-	wantMetric(t, metrics, `smallwebwaf_country_request_bytes_total{country="DE"}`, 6)
-	wantMetric(t, metrics, `smallwebwaf_country_response_bytes_total{country="KP"}`,
+	wantMetric(t, metrics,
+		`smallwebwaf_country_requests_total{country="KP",instance="app"}`, 3)
+	wantMetric(t, metrics,
+		`smallwebwaf_country_requests_total{country="DE",instance="app"}`, 2)
+	wantMetric(t, metrics,
+		`smallwebwaf_country_requests_total{country="other",instance="app"}`, 1)
+	wantMetric(t, metrics,
+		`smallwebwaf_country_list_refusals_total{country="KP",instance="app"}`, 3)
+	wantMetric(t, metrics,
+		`smallwebwaf_country_request_bytes_total{country="KP",instance="app"}`, 0)
+	wantMetric(t, metrics,
+		`smallwebwaf_country_request_bytes_total{country="DE",instance="app"}`, 6)
+	wantMetric(t, metrics,
+		`smallwebwaf_country_response_bytes_total{country="KP",instance="app"}`,
 		float64(3*len("Forbidden\n")))
-	wantMetric(t, metrics, `smallwebwaf_country_response_bytes_total{country="other"}`,
+	wantMetric(t, metrics,
+		`smallwebwaf_country_response_bytes_total{country="other",instance="app"}`,
 		float64(len("hello")))
-	wantNoSeries(t, metrics, `smallwebwaf_country_requests_total{country="FR"}`)
+	wantNoSeries(t, metrics,
+		`smallwebwaf_country_requests_total{country="FR",instance="app"}`)
 
 	// Once FR is busier than DE, it takes DE's place: its series counts
 	// from then on, and DE's is gone.
 	send(fromFR, 3, http.StatusOK)
 
 	metrics = scrape(t, addr)
-	wantMetric(t, metrics, `smallwebwaf_country_requests_total{country="KP"}`, 3)
-	wantMetric(t, metrics, `smallwebwaf_country_requests_total{country="FR"}`, 2)
-	wantMetric(t, metrics, `smallwebwaf_country_requests_total{country="other"}`, 2)
-	wantNoSeries(t, metrics, `smallwebwaf_country_requests_total{country="DE"}`)
-	wantNoSeries(t, metrics, `smallwebwaf_country_request_bytes_total{country="DE"}`)
+	wantMetric(t, metrics,
+		`smallwebwaf_country_requests_total{country="KP",instance="app"}`, 3)
+	wantMetric(t, metrics,
+		`smallwebwaf_country_requests_total{country="FR",instance="app"}`, 2)
+	wantMetric(t, metrics,
+		`smallwebwaf_country_requests_total{country="other",instance="app"}`, 2)
+	wantNoSeries(t, metrics,
+		`smallwebwaf_country_requests_total{country="DE",instance="app"}`)
+	wantNoSeries(t, metrics,
+		`smallwebwaf_country_request_bytes_total{country="DE",instance="app"}`)
 }
 
 func TestMetricsCountGeoJSRequestsAndFailures(t *testing.T) {
@@ -370,16 +388,16 @@ func TestMetricsCountGeoJSRequestsAndFailures(t *testing.T) {
 	deadline := time.Now().Add(waitLimit)
 	metrics := scrape(t, addr)
 
-	for metric(t, metrics, "smallwebwaf_geojs_failures_total") == 0 &&
+	for metric(t, metrics, `smallwebwaf_geojs_failures_total{instance="app"}`) == 0 &&
 		time.Now().Before(deadline) {
 		time.Sleep(pollInterval)
 
 		metrics = scrape(t, addr)
 	}
 
-	wantMetric(t, metrics, "smallwebwaf_geojs_requests_total", 1)
-	wantMetric(t, metrics, "smallwebwaf_geojs_failures_total", 1)
-	wantMetric(t, metrics, "smallwebwaf_geojs_unanswered_total", 1)
+	wantMetric(t, metrics, `smallwebwaf_geojs_requests_total{instance="app"}`, 1)
+	wantMetric(t, metrics, `smallwebwaf_geojs_failures_total{instance="app"}`, 1)
+	wantMetric(t, metrics, `smallwebwaf_geojs_unanswered_total{instance="app"}`, 1)
 }
 
 // keptAnswer returns GeoJS's answer that the client at addr is in
@@ -422,8 +440,9 @@ func (s *sender) scrape(from string) string {
 
 // metric returns the value of series in metrics, which are in the
 // Prometheus text format. series is a name and its labels in the order of
-// their names, such as smallwebwaf_offences_total{kind="limit"}. It fails
-// the test if there is no such series.
+// their names, such as
+// smallwebwaf_offences_total{instance="app",kind="limit"}. It fails the
+// test if there is no such series.
 func metric(t *testing.T, metrics, series string) float64 {
 	t.Helper()
 
@@ -471,7 +490,8 @@ func wantNoSeries(t *testing.T, metrics, series string) {
 func wantLimitHits(t *testing.T, addr, limit string, hits int) {
 	t.Helper()
 
-	series := `smallwebwaf_size_and_time_limit_hits_total{limit="` + limit + `"}`
+	series := `smallwebwaf_size_and_time_limit_hits_total{instance="app",limit="` +
+		limit + `"}`
 	metrics := scrape(t, addr)
 
 	if hits == 0 {
diff --git a/internal/proxy/passthrough_test.go b/internal/proxy/passthrough_test.go
index 5a7f015..c63ceb2 100644
--- a/internal/proxy/passthrough_test.go
+++ b/internal/proxy/passthrough_test.go
@@ -6,7 +6,6 @@ import (
 	"errors"
 	"io"
 	"net/http"
-	"os"
 	"reflect"
 	"slices"
 	"strings"
@@ -123,10 +122,8 @@ func wantAnswer(t *testing.T, got answer, body []byte) {
 func wantRequestFields(t *testing.T, line logLine, host string, sent, received int) {
 	t.Helper()
 
-	hostname, _ := os.Hostname()
-
 	want := withTimings(line, requestlog.Line{
-		Type: requestType, Time: line.Time, Instance: hostname,
+		Type: requestType, Time: line.Time, Instance: "app",
 		ClientIP: localhost, Method: http.MethodPatch, Scheme: plain, Host: host,
 		Path: rawPath, Query: rawQuery, Protocol: protocol,
 		Status: http.StatusTeapot, RequestBytes: int64(sent),
@@ -318,7 +315,7 @@ func TestServerHasTheDefaultLimits(t *testing.T) {
 	server := proxy.New(proxy.Params{
 		Config:     cfg,
 		RequestLog: io.Discard,
-		ProcessLog: requestlog.NewProcessLogger(io.Discard),
+		ProcessLog: requestlog.NewProcessLogger(io.Discard, cfg.InstanceName),
 	})
 
 	if server.Addr != ":8080" || server.MaxHeaderBytes != 28<<10 ||
diff --git a/internal/proxy/proxy.go b/internal/proxy/proxy.go
index 1650cb6..21b1000 100644
--- a/internal/proxy/proxy.go
+++ b/internal/proxy/proxy.go
@@ -88,7 +88,7 @@ type Server struct {
 // applies the timeouts and size limits from then on.
 func New(params Params) *Server {
 	errorLog := slog.NewLogLogger(params.ProcessLog.Handler(), slog.LevelWarn)
-	m := metrics.New(params.Config.MetricsTopN)
+	m := metrics.New(params.Config.MetricsTopN, params.Config.InstanceName)
 	h := &handler{
 		config:     params.Config,
 		requestLog: params.RequestLog,
diff --git a/internal/proxy/proxy_test.go b/internal/proxy/proxy_test.go
index e47ed89..cb6fa66 100644
--- a/internal/proxy/proxy_test.go
+++ b/internal/proxy/proxy_test.go
@@ -217,7 +217,9 @@ func startProxyWithGeoJS(
 
 // startProxyWithClock is startProxyWithGeoJS with requests counted and
 // bans made by the time now tells, and returns the server as well. Unless
-// env sets SWWAF_RULES_DIR, it is an empty directory, of no rules.
+// env sets SWWAF_RULES_DIR, it is an empty directory, of no rules, and
+// unless it sets SWWAF_INSTANCE_NAME, that is app, the label instance of
+// every metric.
 func startProxyWithClock(
 	t *testing.T, appURL, geojsURL string, now func() time.Time,
 	env map[string]string,
@@ -238,7 +240,9 @@ func startProxyWithAlerts(
 ) (string, *output, *proxy.Server, *alerts.Queue) {
 	t.Helper()
 
-	settings := map[string]string{"SWWAF_UPSTREAM_URL": appURL, rulesDir: t.TempDir()}
+	settings := map[string]string{
+		"SWWAF_UPSTREAM_URL": appURL, rulesDir: t.TempDir(), instanceName: "app",
+	}
 	maps.Copy(settings, env)
 
 	cfg, err := config.FromEnvironment(func(name string) (string, bool) {
@@ -251,7 +255,7 @@ func startProxyWithAlerts(
 	}
 
 	out := &output{}
-	processLog := requestlog.NewProcessLogger(out)
+	processLog := requestlog.NewProcessLogger(out, cfg.InstanceName)
 
 	ruleFiles, err := rules.Load(rules.Params{
 		Dir: cfg.RulesDir, Enabled: cfg.RulesEnabled, ProcessLog: processLog,
diff --git a/internal/proxy/rulefiles_test.go b/internal/proxy/rulefiles_test.go
index 7d85882..db27689 100644
--- a/internal/proxy/rulefiles_test.go
+++ b/internal/proxy/rulefiles_test.go
@@ -197,15 +197,15 @@ func TestMetricsCountRuleMatchesAndBansForAnAttack(t *testing.T) {
 
 	metrics := s.scrape(scraper)
 	wantMetric(t, metrics,
-		`smallwebwaf_rule_matches_total{action="block",rule_id="blocked"}`, 1)
+		`smallwebwaf_rule_matches_total{action="block",instance="app",rule_id="blocked"}`, 1)
 	wantMetric(t, metrics,
-		`smallwebwaf_rule_matches_total{action="ban",rule_id="probe"}`, 1)
-	wantMetric(t, metrics, "smallwebwaf_rules_loaded", 2)
-	wantMetric(t, metrics,
-		`smallwebwaf_requests_total{action="rule_blocked",status_class="4xx"}`, 1)
-	wantMetric(t, metrics, `smallwebwaf_bans_made_total{cause="attack"}`, 1)
-	wantMetric(t, metrics, `smallwebwaf_bans_made_total{cause="limit"}`, 0)
-	wantMetric(t, metrics, "smallwebwaf_permanent_bans", 1)
+		`smallwebwaf_rule_matches_total{action="ban",instance="app",rule_id="probe"}`, 1)
+	wantMetric(t, metrics, `smallwebwaf_rules_loaded{instance="app"}`, 2)
+	wantMetric(t, metrics, `smallwebwaf_requests_total{action="rule_blocked",`+
+		`instance="app",status_class="4xx"}`, 1)
+	wantMetric(t, metrics, `smallwebwaf_bans_made_total{cause="attack",instance="app"}`, 1)
+	wantMetric(t, metrics, `smallwebwaf_bans_made_total{cause="limit",instance="app"}`, 0)
+	wantMetric(t, metrics, `smallwebwaf_permanent_bans{instance="app"}`, 1)
 }
 
 // writeRules writes content as a rule file into a new directory, and
diff --git a/internal/requestlog/requestlog.go b/internal/requestlog/requestlog.go
index f7330c2..d18ffb2 100644
--- a/internal/requestlog/requestlog.go
+++ b/internal/requestlog/requestlog.go
@@ -171,8 +171,8 @@ func Milliseconds(d time.Duration) float64 {
 
 // NewProcessLogger returns the logger for the process's own messages:
 // JSON lines on w, marked "type":"process", with the time in the same form
-// as a request line's.
-func NewProcessLogger(w io.Writer) *slog.Logger {
+// as a request line's, and instanceName, SWWAF_INSTANCE_NAME, as instance.
+func NewProcessLogger(w io.Writer, instanceName string) *slog.Logger {
 	handler := slog.NewJSONHandler(w, &slog.HandlerOptions{
 		ReplaceAttr: func(groups []string, attr slog.Attr) slog.Attr {
 			if attr.Key == slog.TimeKey && len(groups) == 0 {
@@ -183,5 +183,5 @@ func NewProcessLogger(w io.Writer) *slog.Logger {
 		},
 	})
 
-	return slog.New(handler).With("type", "process")
+	return slog.New(handler).With("type", "process", "instance", instanceName)
 }
diff --git a/internal/requestlog/requestlog_test.go b/internal/requestlog/requestlog_test.go
index 38303b6..eae737e 100644
--- a/internal/requestlog/requestlog_test.go
+++ b/internal/requestlog/requestlog_test.go
@@ -65,12 +65,12 @@ func TestWriteWritesOneJSONLineMarkedRequest(t *testing.T) {
 	}
 }
 
-func TestProcessLinesAreMarkedProcess(t *testing.T) {
+func TestProcessLinesAreMarkedProcessAndGiveTheInstance(t *testing.T) {
 	t.Parallel()
 
 	var out bytes.Buffer
 
-	requestlog.NewProcessLogger(&out).Info("starting", "version", "v1")
+	requestlog.NewProcessLogger(&out, "fsn1app1/gitea").Info("starting", "version", "v1")
 
 	var fields map[string]any
 
@@ -79,8 +79,9 @@ func TestProcessLinesAreMarkedProcess(t *testing.T) {
 		t.Fatalf("decode %q: %v", out.String(), err)
 	}
 
-	if fields["type"] != "process" || fields["msg"] != "starting" ||
-		fields["level"] != "INFO" || fields["version"] != "v1" {
+	if fields["type"] != "process" || fields["instance"] != "fsn1app1/gitea" ||
+		fields["msg"] != "starting" || fields["level"] != "INFO" ||
+		fields["version"] != "v1" {
 		t.Errorf("process line %v", fields)
 	}
 
diff --git a/internal/smallwebwaf/smallwebwaf.go b/internal/smallwebwaf/smallwebwaf.go
index 94113fb..b2c1fda 100644
--- a/internal/smallwebwaf/smallwebwaf.go
+++ b/internal/smallwebwaf/smallwebwaf.go
@@ -68,7 +68,8 @@ func Main(version string) int {
 // requests until ctx is done. It returns the process's exit status, 1
 // when smallwebwaf cannot start.
 func Run(ctx context.Context, params Params) int {
-	processLog := requestlog.NewProcessLogger(params.Stdout)
+	processLog := requestlog.NewProcessLogger(params.Stdout,
+		config.InstanceName(params.LookupEnv))
 
 	cfg, err := config.FromEnvironment(params.LookupEnv)
 	if err != nil {
@@ -86,7 +87,7 @@ func Run(ctx context.Context, params Params) int {
 	if cfg.LogRemoteURL != nil {
 		remote = newRemoteLogSender(cfg)
 		stdout = io.MultiWriter(params.Stdout, remote)
-		processLog = requestlog.NewProcessLogger(stdout)
+		processLog = requestlog.NewProcessLogger(stdout, cfg.InstanceName)
 
 		stopSending := startSending(ctx, remote, processLog)
 		defer stopSending()
diff --git a/internal/smallwebwaf/smallwebwaf_test.go b/internal/smallwebwaf/smallwebwaf_test.go
index e0e9099..bcc097b 100644
--- a/internal/smallwebwaf/smallwebwaf_test.go
+++ b/internal/smallwebwaf/smallwebwaf_test.go
@@ -38,10 +38,14 @@ const (
 	stateCounterInterval = "SWWAF_STATE_COUNTER_INTERVAL"
 	rateLimitPerDay      = "SWWAF_RATE_LIMIT_PER_DAY"
 	rulesDir             = "SWWAF_RULES_DIR"
+	instanceName         = "SWWAF_INSTANCE_NAME"
 	adminToken           = "SWWAF_ADMIN_TOKEN"   //nolint:gosec // the setting's name
 	metricsToken         = "SWWAF_METRICS_TOKEN" //nolint:gosec // the setting's name
 	// adminSecret is the SWWAF_ADMIN_TOKEN the tests set.
 	adminSecret = "fedcba9876543210fedcba9876543210"
+	// instance is the SWWAF_INSTANCE_NAME the tests set where they look at
+	// it.
+	instance = "fsn1app1/gitea"
 	// greeting is what the tests' app answers.
 	greeting = "hello from the app"
 )
@@ -118,7 +122,10 @@ func TestInvalidSettingStopsTheStart(t *testing.T) {
 
 	out := &output{}
 
-	status := run(t.Context(), map[string]string{"SWWAF_REQUEST_MAX_BYTES": "lots"}, out)
+	status := run(t.Context(), map[string]string{
+		"SWWAF_REQUEST_MAX_BYTES": "lots",
+		instanceName:              instance,
+	}, out)
 	if status != 1 {
 		t.Errorf("exit status %d, want 1", status)
 	}
@@ -126,7 +133,8 @@ func TestInvalidSettingStopsTheStart(t *testing.T) {
 	line := out.line(t, "msg", "invalid setting")
 	message, _ := line["error"].(string)
 
-	if line["type"] != "process" || line["level"] != "ERROR" ||
+	if line["type"] != "process" || line["instance"] != instance ||
+		line["level"] != "ERROR" ||
 		!strings.HasPrefix(message, "SWWAF_REQUEST_MAX_BYTES: ") {
 		t.Errorf("start refused with %v", line)
 	}
@@ -231,6 +239,46 @@ func TestServesUntilToldToStop(t *testing.T) {
 	out.line(t, "msg", "stopped")
 }
 
+func TestEveryLogLineAndMetricCarriesTheInstanceName(t *testing.T) {
+	t.Parallel()
+
+	const token = "0123456789abcdef0123456789abcdef"
+
+	env := map[string]string{
+		listenAddr:   localhost + ":0",
+		upstreamURL:  startApp(t),
+		stateDir:     t.TempDir(),
+		rulesDir:     t.TempDir(),
+		metricsToken: token,
+		instanceName: instance,
+	}
+
+	var metrics string
+
+	out := runUntilStopped(t, env, func(url string) {
+		metrics = metricsText(t, url+"_smallwebwaf/metrics", token)
+	})
+
+	// The process's lines from its start to its stop, and the request's.
+	for line := range strings.Lines(out.text()) {
+		var fields map[string]any
+
+		err := json.Unmarshal([]byte(line), &fields)
+		if err != nil || fields["instance"] != instance {
+			t.Errorf("line %q (%v), want instance %s", line, err, instance)
+		}
+	}
+
+	// Each series, Go's and the process's included; the other lines are
+	// the comments.
+	for line := range strings.Lines(metrics) {
+		if !strings.HasPrefix(line, "#") &&
+			!strings.Contains(line, `instance="fsn1app1/gitea"`) {
+			t.Errorf("series %q, without instance=\"fsn1app1/gitea\"", line)
+		}
+	}
+}
+
 func TestStateKeptAcrossRestarts(t *testing.T) {
 	t.Parallel()
 
@@ -519,6 +567,7 @@ func TestStalledRemoteLogEndpointHoldsUpNoRequest(t *testing.T) {
 		"SWWAF_LOG_REMOTE_URL":    "syslog+tls://" + endpoint.Addr().String(),
 		"SWWAF_LOG_REMOTE_BUFFER": "1",
 		metricsToken:              token,
+		instanceName:              instance,
 	}
 
 	out := runUntilStopped(t, env, func(url string) {
@@ -528,16 +577,17 @@ func TestStalledRemoteLogEndpointHoldsUpNoRequest(t *testing.T) {
 		// last.
 		metrics := metricsText(t, url+"_smallwebwaf/metrics", token)
 		for _, series := range []string{
-			"smallwebwaf_remote_log_lines_sent_total 0",
-			"smallwebwaf_remote_log_buffer_depth 1",
+			`smallwebwaf_remote_log_lines_sent_total{instance="fsn1app1/gitea"} 0`,
+			`smallwebwaf_remote_log_buffer_depth{instance="fsn1app1/gitea"} 1`,
 		} {
 			if !strings.Contains(metrics, "\n"+series+"\n") {
 				t.Errorf("no %q in the metrics:\n%s", series, metrics)
 			}
 		}
 
-		if strings.Contains(metrics, "\nsmallwebwaf_remote_log_lines_dropped_total 0\n") ||
-			!strings.Contains(metrics, "\nsmallwebwaf_remote_log_lines_dropped_total ") {
+		const dropped = "\nsmallwebwaf_remote_log_lines_dropped_total" +
+			`{instance="fsn1app1/gitea"} `
+		if strings.Contains(metrics, dropped+"0\n") || !strings.Contains(metrics, dropped) {
 			t.Errorf("no line dropped in the metrics:\n%s", metrics)
 		}
 
@@ -547,6 +597,13 @@ func TestStalledRemoteLogEndpointHoldsUpNoRequest(t *testing.T) {
 	})
 
 	out.line(t, "type", "request")
+
+	// While the lines are sent, the process's lines give the instance name
+	// too.
+	line := out.line(t, "msg", "starting")
+	if line["instance"] != instance {
+		t.Errorf("start logged with instance %v, want %s", line["instance"], instance)
+	}
 }
 
 func TestBanIsAlertedAndAnAlertNotSentIsKeptAcrossARestart(t *testing.T) {
@@ -569,6 +626,7 @@ func TestBanIsAlertedAndAnAlertNotSentIsKeptAcrossARestart(t *testing.T) {
 		rulesDir:                      rules,
 		"SWWAF_ALERT_WEBHOOK_URL":     webhook.url,
 		"SWWAF_ALERT_WEBHOOK_HEADERS": "Authorization:Bearer " + adminSecret,
+		instanceName:                  instance,
 	}
 
 	runUntilStopped(t, env, func(url string) {
@@ -622,19 +680,13 @@ func TestBanIsAlertedAndAnAlertNotSentIsKeptAcrossARestart(t *testing.T) {
 	runUntilStopped(t, env, func(url string) {
 		webhook.waitFor(t, "permanent_ban", true)
 
-		// As long as that takes, so that a slow test process cannot fail
-		// the test.
-		const sent = "\nsmallwebwaf_alerts_sent_total{destination=\"webhook\"} 1\n"
+		const ofWebhook = `{destination="webhook",instance="fsn1app1/gitea"}`
 
-		metrics := metricsText(t, url+"_smallwebwaf/metrics", token)
-		for !strings.Contains(metrics, sent) {
-			time.Sleep(pollInterval)
-
-			metrics = metricsText(t, url+"_smallwebwaf/metrics", token)
-		}
+		metrics := metricsWith(t, url+"_smallwebwaf/metrics", token,
+			"\nsmallwebwaf_alerts_sent_total"+ofWebhook+" 1\n")
 
 		for _, series := range []string{"failed", "suppressed", "dropped"} {
-			zero := "\nsmallwebwaf_alerts_" + series + "_total{destination=\"webhook\"} 0\n"
+			zero := "\nsmallwebwaf_alerts_" + series + "_total" + ofWebhook + " 0\n"
 			if !strings.Contains(metrics, zero) {
 				t.Errorf("no %q in the metrics:\n%s", zero, metrics)
 			}
@@ -662,7 +714,7 @@ func TestBanIsAlertedToSlackAndNtfy(t *testing.T) {
 		// The metrics are read from 127.0.0.1, which no limit counts.
 		"SWWAF_ALLOW_NETS":              localhost + "/32",
 		metricsToken:                    token,
-		"SWWAF_INSTANCE_NAME":           "fsn1app1/gitea",
+		instanceName:                    instance,
 		"SWWAF_ALERT_SLACK_WEBHOOK_URL": slack.url,
 		"SWWAF_ALERT_NTFY_URL":          ntfy.url,
 		"SWWAF_ALERT_NTFY_TOKEN":        ntfyToken,
@@ -698,17 +750,13 @@ func TestBanIsAlertedToSlackAndNtfy(t *testing.T) {
 		// webhook, which is not set. As long as that takes, so that a slow
 		// test process cannot fail the test.
 		sent := []string{
-			"\nsmallwebwaf_alerts_sent_total{destination=\"slack\"} 1\n",
-			"\nsmallwebwaf_alerts_sent_total{destination=\"ntfy\"} 1\n",
-		}
-
-		metrics := metricsText(t, url+"_smallwebwaf/metrics", token)
-		for !strings.Contains(metrics, sent[0]) || !strings.Contains(metrics, sent[1]) {
-			time.Sleep(pollInterval)
-
-			metrics = metricsText(t, url+"_smallwebwaf/metrics", token)
+			"\nsmallwebwaf_alerts_sent_total{destination=\"slack\"," +
+				"instance=\"fsn1app1/gitea\"} 1\n",
+			"\nsmallwebwaf_alerts_sent_total{destination=\"ntfy\"," +
+				"instance=\"fsn1app1/gitea\"} 1\n",
 		}
 
+		metrics := metricsWith(t, url+"_smallwebwaf/metrics", token, sent...)
 		if strings.Contains(metrics, `destination="webhook"`) {
 			t.Errorf("the metrics give the webhook:\n%s", metrics)
 		}
@@ -974,6 +1022,26 @@ func metricsText(t *testing.T, url, token string) string {
 	return string(body)
 }
 
+// metricsWith asks for the metrics at url with token until they hold each
+// of series, as long as that takes, so that a slow test process cannot
+// fail the test, and returns them.
+func metricsWith(t *testing.T, url, token string, series ...string) string {
+	t.Helper()
+
+	for {
+		metrics := metricsText(t, url, token)
+
+		missing := slices.ContainsFunc(series, func(one string) bool {
+			return !strings.Contains(metrics, one)
+		})
+		if !missing {
+			return metrics
+		}
+
+		time.Sleep(pollInterval)
+	}
+}
+
 // askAsAdmin sends a request with method to url, with body and
 // adminSecret, and checks that it is answered 200.
 func askAsAdmin(t *testing.T, method, url, body string) {
diff --git a/internal/state/state_test.go b/internal/state/state_test.go
index 1accc7d..a280ec6 100644
--- a/internal/state/state_test.go
+++ b/internal/state/state_test.go
@@ -736,8 +736,8 @@ func TestWritesAreCountedInTheMetrics(t *testing.T) {
 	}
 
 	const (
-		ofBans    = `{file="bans.json"}`
-		ofClients = `{file="clients.json"}`
+		ofBans    = `{file="bans.json",instance="app"}`
+		ofClients = `{file="clients.json",instance="app"}`
 	)
 
 	got := scrape(t, params)
@@ -1162,7 +1162,7 @@ func TestEditsTakenInAreCountedInTheMetrics(t *testing.T) {
 	wantTakenIn(t, lines, dir, bansJSON)
 
 	wantMetric(t, scrape(t, params),
-		`smallwebwaf_state_file_edits_taken_in_total{file="bans.json"}`, 2)
+		`smallwebwaf_state_file_edits_taken_in_total{file="bans.json",instance="app"}`, 2)
 }
 
 func TestEditTakenInByAWriteIsLoggedAsWatchLogsIt(t *testing.T) {
@@ -1227,7 +1227,7 @@ func TestEditsSetAsideAreCountedInTheMetrics(t *testing.T) {
 	}
 
 	wantMetric(t, scrape(t, params),
-		`smallwebwaf_state_file_edits_set_aside_total{file="bans.json"}`, 1)
+		`smallwebwaf_state_file_edits_set_aside_total{file="bans.json",instance="app"}`, 1)
 }
 
 func TestDirectoryThatCannotBeWatchedIsLogged(t *testing.T) {
@@ -1262,7 +1262,7 @@ func midnight() time.Time {
 // hour, are never sent.
 func newParams(dir string) state.Params {
 	discard := slog.New(slog.DiscardHandler)
-	m := metrics.New(1)
+	m := metrics.New(1, "app")
 
 	return state.Params{
 		Dir:             dir,
@@ -1647,8 +1647,8 @@ func scrape(t *testing.T, params state.Params) string {
 }
 
 // metric returns the value of series in text, the metrics, such as
-// smallwebwaf_state_file_writes_total{file="bans.json"}, or fails the test
-// if there is no such series.
+// smallwebwaf_state_file_writes_total{file="bans.json",instance="app"}, or
+// fails the test if there is no such series.
 func metric(t *testing.T, text, series string) float64 {
 	t.Helper()
 
@@ -1677,7 +1677,7 @@ func wantWriteFailed(t *testing.T, params state.Params, name string) {
 	t.Helper()
 
 	got := scrape(t, params)
-	file := `{file="` + name + `"}`
+	file := `{file="` + name + `",instance="app"}`
 
 	wantMetric(t, got, "smallwebwaf_state_file_writes_total"+file, 1)
 	wantMetric(t, got, "smallwebwaf_state_file_write_failures_total"+file, 1)