Instance name on process log lines and every metric (closes #91)

Process log lines carry instance, as request lines do; the instance name
is read before the other settings, so the line saying a setting is
invalid carries it too. Every metric, Go's and the process's included,
carries the label instance, set once on the registry. README.md says so,
and that Prometheus keeps it as exported_instance unless the scrape sets
honor_labels. An instance name that is not valid UTF-8 stops the start,
as the metrics library panics on such a label.

Tests that read metrics expect the label; one helper replaces the alert
tests' loops that wait for them.

Judgement call: the label is named instance, as in the log lines and
alerts, although Prometheus gives each target a label of that name.

Model: opus-5-5

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,