diff --git a/internal/middleware/export_test.go b/internal/middleware/export_test.go
index 31abd52..d0033d4 100644
--- a/internal/middleware/export_test.go
+++ b/internal/middleware/export_test.go
@@ -4,8 +4,28 @@ import (
 	"context"
 	"net/http"
 	"time"
+
+	httpmetrics "github.com/slok/go-http-metrics/metrics"
 )
 
+// MetricsMiddlewareForTest builds the metrics recording middleware
+// against a caller-supplied recorder, so a test can gather from its
+// own Prometheus registry rather than the process-wide default one
+// that Middleware.Metrics uses.
+func MetricsMiddlewareForTest(
+	rec httpmetrics.Recorder,
+) func(http.Handler) http.Handler {
+	return metricsMiddleware(rec)
+}
+
+// UnmatchedRouteConst exposes the sentinel that stands in for a
+// request matching no route pattern.
+const UnmatchedRouteConst = unmatchedRoute
+
+// InflightHandlerConst exposes the fixed handler label on the
+// inflight gauge.
+const InflightHandlerConst = inflightHandler
+
 // NewLoggingResponseWriterForTest wraps newLoggingResponseWriter
 // for use in external test packages.
 func NewLoggingResponseWriterForTest(
diff --git a/internal/middleware/metrics.go b/internal/middleware/metrics.go
new file mode 100644
index 0000000..14d36a2
--- /dev/null
+++ b/internal/middleware/metrics.go
@@ -0,0 +1,135 @@
+package middleware
+
+import (
+	"context"
+	"net/http"
+	"time"
+
+	"github.com/go-chi/chi"
+	httpmetrics "github.com/slok/go-http-metrics/metrics"
+	prommetrics "github.com/slok/go-http-metrics/metrics/prometheus"
+	ghmm "github.com/slok/go-http-metrics/middleware"
+	"github.com/slok/go-http-metrics/middleware/std"
+)
+
+// inflightHandler is the fixed `handler` label on
+// http_requests_inflight, the one HTTP metric here that cannot carry
+// a route pattern.
+//
+// The gauge is incremented before the wrapped handler runs and
+// decremented after it returns, and the pattern only exists between
+// those two moments. Deriving the label from the route would
+// therefore increment one series and decrement another, leaving every
+// pattern permanently off by the number of requests it served — a
+// broken gauge, on top of the per-path cardinality this file exists
+// to remove. So the gauge is deliberately aggregate: one series,
+// counting the requests in flight across the whole service.
+const inflightHandler = "(all)"
+
+// routePatternID is the `handler` label for a request: the chi route
+// pattern, never the concrete path.
+//
+// The pattern is what bounds the label's domain to the routes the
+// service registers. The path does not bound it at all — every byte
+// after /webhook/ is client-chosen, so labelling by path lets any
+// unauthenticated client mint permanent series at will, and publishes
+// the entrypoint UUID (the receiver's only credential) in the scrape
+// while doing it.
+//
+// chi populates the route context during routeHTTP, so this is only
+// valid once routing has run. Every caller below is on the recording
+// side of the middleware, which go-http-metrics defers until after
+// the wrapped handler returns.
+func routePatternID(ctx context.Context) string {
+	if rc := chi.RouteContext(ctx); rc != nil {
+		if pattern := rc.RoutePattern(); pattern != "" {
+			return pattern
+		}
+	}
+
+	return unmatchedRoute
+}
+
+// routePatternRecorder wraps a go-http-metrics recorder and replaces
+// the handler id on every observation with the request's route
+// pattern.
+//
+// This is the seam that makes the pattern usable at all. The metrics
+// middleware is global (see Server.setupGlobalMiddleware), so it is
+// entered before chi has matched anything, and go-http-metrics fixes
+// its handler id up front — passing the pattern in as that id is not
+// possible, and leaving the id empty makes the library substitute the
+// raw URL path, which is the defect. What the library does hand over
+// is the request context, unchanged, on each recorder call; that
+// context carries the same *chi.Context pointer routing mutates in
+// place, and the duration and size calls happen after the wrapped
+// handler has returned. Reading the pattern there is what the access
+// log already does in accessLogURL.
+//
+// Recording after the whole chain returns is also what makes this
+// hold for requests the route-level receiver rate limiter rejects.
+// Those never reach a handler, but chi has already matched the route
+// by the time the limiter runs, so their 429s land on the pattern
+// like any other response.
+type routePatternRecorder struct {
+	inner httpmetrics.Recorder
+}
+
+func (r routePatternRecorder) ObserveHTTPRequestDuration(
+	ctx context.Context,
+	props httpmetrics.HTTPReqProperties,
+	duration time.Duration,
+) {
+	props.ID = routePatternID(ctx)
+	r.inner.ObserveHTTPRequestDuration(ctx, props, duration)
+}
+
+func (r routePatternRecorder) ObserveHTTPResponseSize(
+	ctx context.Context,
+	props httpmetrics.HTTPReqProperties,
+	sizeBytes int64,
+) {
+	props.ID = routePatternID(ctx)
+	r.inner.ObserveHTTPResponseSize(ctx, props, sizeBytes)
+}
+
+func (r routePatternRecorder) AddInflightRequests(
+	ctx context.Context,
+	props httpmetrics.HTTPProperties,
+	quantity int,
+) {
+	props.ID = inflightHandler
+	r.inner.AddInflightRequests(ctx, props, quantity)
+}
+
+var _ httpmetrics.Recorder = routePatternRecorder{}
+
+// Metrics returns middleware that records Prometheus HTTP metrics on
+// the default registry, which is the one the /metrics route gathers.
+func (s *Middleware) Metrics() func(http.Handler) http.Handler {
+	return metricsMiddleware(
+		prommetrics.NewRecorder(prommetrics.Config{}),
+	)
+}
+
+// metricsMiddleware builds the recording middleware against a given
+// recorder, so tests can gather from a registry of their own instead
+// of the process-wide default.
+func metricsMiddleware(
+	rec httpmetrics.Recorder,
+) func(http.Handler) http.Handler {
+	mdlw := ghmm.New(ghmm.Config{
+		Recorder: routePatternRecorder{inner: rec},
+	})
+
+	return func(next http.Handler) http.Handler {
+		// The handler id is unmatchedRoute rather than "" so that
+		// the client-chosen URL path never enters the metrics
+		// pipeline at all: an empty id is the library's signal to
+		// substitute it. routePatternRecorder overwrites this value
+		// on every observation, so it is reachable only if that
+		// decorator is removed — in which case the metrics collapse
+		// to one series instead of leaking again.
+		return std.Handler(unmatchedRoute, mdlw, next)
+	}
+}
diff --git a/internal/middleware/metrics_test.go b/internal/middleware/metrics_test.go
new file mode 100644
index 0000000..39776f2
--- /dev/null
+++ b/internal/middleware/metrics_test.go
@@ -0,0 +1,434 @@
+package middleware_test
+
+import (
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/go-chi/chi"
+	"github.com/google/uuid"
+	"github.com/prometheus/client_golang/prometheus"
+	dto "github.com/prometheus/client_model/go"
+	prommetrics "github.com/slok/go-http-metrics/metrics/prometheus"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/middleware"
+)
+
+const (
+	// metricsProbePaths is how many distinct receiver paths each
+	// cardinality assertion drives. The defect these tests pin cost
+	// roughly 26 permanent series per distinct path, so a couple of
+	// hundred puts a regression thousands of series over the bound
+	// rather than leaving it to a rounding argument.
+	metricsProbePaths = 250
+
+	// receiverRoutePattern is the one handler label every receiver
+	// request must produce, however the client varies the path.
+	receiverRoutePattern = "/webhook/{uuid}"
+
+	// okRoute is a static route used to pin that the response-writer
+	// interceptor still reports status and size after the handler id
+	// stopped coming from the URL.
+	okRoute = "/ok"
+
+	// okBody is what okRoute writes, so the recorded response size is
+	// a number the test knows.
+	okBody = "ok"
+
+	// generousReceiverLimit is a per-entrypoint receiver limit high
+	// enough that no probe in this file trips the limiter unless it
+	// means to.
+	generousReceiverLimit = 100000
+
+	// tightReceiverLimit forces the receiver's aggregate limiter to
+	// reject: the aggregate ceiling is ten times this, so a probe of
+	// metricsProbePaths requests spends it many times over.
+	tightReceiverLimit = 1
+)
+
+// metricsTestRouter builds a router whose middleware ordering mirrors
+// the real server's: the metrics recorder is GLOBAL, installed by
+// Server.setupGlobalMiddleware before chi has matched anything, and
+// the receiver rate limiter is ROUTE-LEVEL, installed by
+// Server.setupWebhookRoutes inside it. That ordering is the whole
+// defect, so a test that flattens it would prove nothing.
+//
+// The recorder writes to a registry of the test's own rather than the
+// process-wide default one, so each test observes only its own
+// traffic.
+func metricsTestRouter(
+	t *testing.T,
+	receiverLimit int,
+) (http.Handler, *prometheus.Registry) {
+	t.Helper()
+
+	log := slog.New(slog.DiscardHandler)
+	cfg := &config.Config{
+		Environment:       "prod",
+		ReceiverRateLimit: receiverLimit,
+	}
+	m := middleware.NewForTest(
+		log, cfg, newTestSessionManager(cfg, log, nil),
+	)
+
+	reg := prometheus.NewRegistry()
+	rec := prommetrics.NewRecorder(prommetrics.Config{Registry: reg})
+
+	r := chi.NewRouter()
+	r.Use(middleware.MetricsMiddlewareForTest(rec))
+
+	r.Get(okRoute, func(w http.ResponseWriter, _ *http.Request) {
+		_, _ = w.Write([]byte(okBody))
+	})
+
+	// The real receiver answers 404 for a UUID naming no stored
+	// entrypoint, which is what every invented path here is.
+	r.With(m.ReceiverRateLimit()).HandleFunc(
+		receiverRoutePattern,
+		func(w http.ResponseWriter, _ *http.Request) {
+			w.WriteHeader(http.StatusNotFound)
+		},
+	)
+
+	return r, reg
+}
+
+// drivePaths sends one POST per supplied path and returns how many
+// responses carried each status code.
+func drivePaths(
+	t *testing.T,
+	h http.Handler,
+	paths []string,
+) map[int]int {
+	t.Helper()
+
+	codes := make(map[int]int)
+
+	for _, p := range paths {
+		req := httptest.NewRequestWithContext(
+			t.Context(), http.MethodPost, p, nil,
+		)
+		w := httptest.NewRecorder()
+		h.ServeHTTP(w, req)
+		codes[w.Code]++
+	}
+
+	return codes
+}
+
+// receiverPaths returns n distinct /webhook/ paths, each naming a
+// fresh UUID exactly as an unauthenticated flood would.
+func receiverPaths(n int) []string {
+	paths := make([]string, 0, n)
+
+	for range n {
+		paths = append(paths, "/webhook/"+uuid.NewString())
+	}
+
+	return paths
+}
+
+// gatherMetrics returns the registry's current families, failing the
+// test if gathering does.
+func gatherMetrics(
+	t *testing.T,
+	reg *prometheus.Registry,
+) []*dto.MetricFamily {
+	t.Helper()
+
+	families, err := reg.Gather()
+	require.NoError(t, err)
+
+	return families
+}
+
+// labelValue returns the named label from a gathered metric.
+func labelValue(m *dto.Metric, name string) string {
+	for _, pair := range m.GetLabel() {
+		if pair.GetName() == name {
+			return pair.GetValue()
+		}
+	}
+
+	return ""
+}
+
+// handlerLabels returns the set of distinct `handler` label values
+// across every gathered series.
+func handlerLabels(families []*dto.MetricFamily) map[string]struct{} {
+	seen := make(map[string]struct{})
+
+	for _, fam := range families {
+		for _, m := range fam.GetMetric() {
+			seen[labelValue(m, "handler")] = struct{}{}
+		}
+	}
+
+	return seen
+}
+
+// seriesCount is the number of distinct label sets held across every
+// family: the quantity that grew without bound and was never
+// reclaimed.
+func seriesCount(families []*dto.MetricFamily) int {
+	total := 0
+
+	for _, fam := range families {
+		total += len(fam.GetMetric())
+	}
+
+	return total
+}
+
+// keys returns the members of a set, for assertion messages.
+func keys(set map[string]struct{}) []string {
+	out := make([]string, 0, len(set))
+
+	for k := range set {
+		out = append(out, k)
+	}
+
+	return out
+}
+
+// TestMetrics_DistinctReceiverPathsMintOneLabelSet is the direct
+// assertion the issue asks for: N requests to N distinct
+// /webhook/<uuid> paths must produce exactly ONE handler label, the
+// route pattern. Before the fix this produced N of them.
+func TestMetrics_DistinctReceiverPathsMintOneLabelSet(t *testing.T) {
+	t.Parallel()
+
+	h, reg := metricsTestRouter(t, generousReceiverLimit)
+
+	paths := receiverPaths(metricsProbePaths)
+	codes := drivePaths(t, h, paths)
+	require.Equal(
+		t, metricsProbePaths, codes[http.StatusNotFound],
+		"every invented UUID should have reached the receiver",
+	)
+
+	families := gatherMetrics(t, reg)
+	labels := handlerLabels(families)
+
+	assert.Equal(
+		t,
+		map[string]struct{}{
+			receiverRoutePattern:            {},
+			middleware.InflightHandlerConst: {},
+		},
+		labels,
+		"receiver traffic must collapse onto the route pattern",
+	)
+
+	// The scrape must not republish the UUIDs it was driven with.
+	// They are the receiver's only credential.
+	for _, p := range paths {
+		id := strings.TrimPrefix(p, "/webhook/")
+		for label := range labels {
+			assert.NotContains(
+				t, label, id,
+				"an entrypoint UUID reached a metrics label",
+			)
+		}
+	}
+}
+
+// TestMetrics_SeriesCountIsFlatUnderAFlood pins the property the
+// issue measured against a live instance: driving thousands more
+// distinct paths must not add series. The first batch establishes
+// every label set the route can produce; the second must land on
+// exactly those.
+func TestMetrics_SeriesCountIsFlatUnderAFlood(t *testing.T) {
+	t.Parallel()
+
+	h, reg := metricsTestRouter(t, generousReceiverLimit)
+
+	drivePaths(t, h, receiverPaths(metricsProbePaths))
+	before := seriesCount(gatherMetrics(t, reg))
+
+	drivePaths(t, h, receiverPaths(metricsProbePaths*4))
+	after := seriesCount(gatherMetrics(t, reg))
+
+	assert.Equal(
+		t, before, after,
+		"a flood of distinct paths must not mint series",
+	)
+}
+
+// TestMetrics_RateLimitedRequestsCarryTheRoutePattern covers the
+// majority case: most of the leaked series were 429s. Those requests
+// never reach a handler, so they take a different path through the
+// stack — but chi has already matched the route by the time the
+// route-level limiter rejects them, and the recording happens after
+// the whole chain returns, so they must land on the pattern too.
+func TestMetrics_RateLimitedRequestsCarryTheRoutePattern(t *testing.T) {
+	t.Parallel()
+
+	h, reg := metricsTestRouter(t, tightReceiverLimit)
+
+	codes := drivePaths(t, h, receiverPaths(metricsProbePaths))
+	require.Positive(
+		t, codes[http.StatusTooManyRequests],
+		"the probe must actually exhaust the aggregate limiter",
+	)
+
+	families := gatherMetrics(t, reg)
+
+	assert.Equal(
+		t,
+		map[string]struct{}{
+			receiverRoutePattern:            {},
+			middleware.InflightHandlerConst: {},
+		},
+		handlerLabels(families),
+		"rejected requests must collapse onto the route pattern",
+	)
+
+	rejected := 0
+
+	for _, fam := range families {
+		for _, m := range fam.GetMetric() {
+			if labelValue(m, "code") != "429" {
+				continue
+			}
+
+			rejected++
+
+			assert.Equal(
+				t, receiverRoutePattern,
+				labelValue(m, "handler"),
+				"a 429 series carried a non-pattern handler",
+			)
+		}
+	}
+
+	assert.Positive(
+		t, rejected, "no 429 series was recorded at all",
+	)
+}
+
+// TestMetrics_UnmatchedPathsCollapseToTheSentinel decides and pins the
+// unmatched-route case. A path matching no route has no pattern, so
+// it carries the same fixed sentinel the access log uses. Without
+// that, an unmatched flood leaks exactly as the receiver did.
+func TestMetrics_UnmatchedPathsCollapseToTheSentinel(t *testing.T) {
+	t.Parallel()
+
+	h, reg := metricsTestRouter(t, generousReceiverLimit)
+
+	paths := make([]string, 0, metricsProbePaths)
+
+	for i := range metricsProbePaths {
+		id := uuid.NewString()
+
+		// Two shapes: one matching no prefix at all, and one under
+		// the receiver prefix but with a segment count the pattern
+		// cannot match.
+		if i%2 == 0 {
+			paths = append(paths, "/"+id)
+		} else {
+			paths = append(paths, "/webhook/"+id+"/"+id)
+		}
+	}
+
+	codes := drivePaths(t, h, paths)
+	require.Equal(
+		t, metricsProbePaths, codes[http.StatusNotFound],
+		"every probe path should have gone unmatched",
+	)
+
+	labels := handlerLabels(gatherMetrics(t, reg))
+
+	assert.Equal(
+		t,
+		map[string]struct{}{
+			middleware.UnmatchedRouteConst:  {},
+			middleware.InflightHandlerConst: {},
+		},
+		labels,
+		"unmatched paths must collapse onto one sentinel, got %v",
+		keys(labels),
+	)
+}
+
+// TestMetrics_InflightGaugeIsAggregateAndBalanced pins the one metric
+// that cannot carry a pattern. It is incremented before routing and
+// decremented after, so it gets a fixed label -- and the two calls
+// must therefore agree, leaving the gauge at zero once the traffic
+// has drained rather than stuck above it.
+func TestMetrics_InflightGaugeIsAggregateAndBalanced(t *testing.T) {
+	t.Parallel()
+
+	h, reg := metricsTestRouter(t, generousReceiverLimit)
+
+	drivePaths(t, h, receiverPaths(metricsProbePaths))
+
+	var inflight []*dto.Metric
+
+	for _, fam := range gatherMetrics(t, reg) {
+		if strings.HasSuffix(fam.GetName(), "requests_inflight") {
+			inflight = fam.GetMetric()
+		}
+	}
+
+	require.Len(
+		t, inflight, 1,
+		"the inflight gauge must hold exactly one series",
+	)
+	assert.Equal(
+		t, middleware.InflightHandlerConst,
+		labelValue(inflight[0], "handler"),
+	)
+	assert.InDelta(
+		t, 0.0, inflight[0].GetGauge().GetValue(), 0.0,
+		"the gauge must balance back to zero",
+	)
+}
+
+// TestMetrics_StatusAndSizeStillRecorded guards the response-writer
+// interceptor the recording middleware wraps around every request.
+// The handler label changed; what the interceptor reports must not
+// have.
+func TestMetrics_StatusAndSizeStillRecorded(t *testing.T) {
+	t.Parallel()
+
+	h, reg := metricsTestRouter(t, generousReceiverLimit)
+
+	req := httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, okRoute, nil,
+	)
+	w := httptest.NewRecorder()
+	h.ServeHTTP(w, req)
+
+	require.Equal(t, http.StatusOK, w.Code)
+	require.Equal(t, okBody, w.Body.String())
+
+	var size *dto.Metric
+
+	for _, fam := range gatherMetrics(t, reg) {
+		if !strings.HasSuffix(fam.GetName(), "response_size_bytes") {
+			continue
+		}
+
+		for _, m := range fam.GetMetric() {
+			if labelValue(m, "handler") == okRoute {
+				size = m
+			}
+		}
+	}
+
+	require.NotNil(
+		t, size, "no response size series for the static route",
+	)
+	assert.Equal(t, "200", labelValue(size, "code"))
+	assert.Equal(t, uint64(1), size.GetHistogram().GetSampleCount())
+	assert.InDelta(
+		t, float64(len(okBody)),
+		size.GetHistogram().GetSampleSum(), 0.0,
+		"the interceptor must still count written bytes",
+	)
+}
diff --git a/internal/middleware/middleware.go b/internal/middleware/middleware.go
index fe76fa0..98140fb 100644
--- a/internal/middleware/middleware.go
+++ b/internal/middleware/middleware.go
@@ -13,9 +13,6 @@ import (
 	"github.com/go-chi/chi"
 	"github.com/go-chi/chi/middleware"
 	"github.com/go-chi/cors"
-	metrics "github.com/slok/go-http-metrics/metrics/prometheus"
-	ghmm "github.com/slok/go-http-metrics/middleware"
-	"github.com/slok/go-http-metrics/middleware/std"
 	"go.uber.org/fx"
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/globals"
@@ -29,10 +26,15 @@ const (
 	// preflight response can be cached.
 	corsMaxAge = 300
 
-	// unmatchedRoute is logged in the access log's url field when a
-	// redirected or rejected request matched no route pattern at
-	// all. Every byte of such a path is client-chosen, so none of it
-	// is logged.
+	// unmatchedRoute stands in for a request that matched no route
+	// pattern at all. Every byte of such a path is client-chosen, so
+	// none of it is kept.
+	//
+	// It is the access log's url field on a redirected or rejected
+	// request, and it is the metrics `handler` label on the same
+	// request; see metrics.go. Both surfaces are written once per
+	// request from a path the client picks, so both have to collapse
+	// the unmatched case into one fixed value.
 	unmatchedRoute = "(unmatched)"
 
 	// redactedQuery stands in for the query string on the access log
@@ -438,17 +440,6 @@ func (s *Middleware) RequireAuth() func(http.Handler) http.Handler {
 	}
 }
 
-// Metrics returns middleware that records Prometheus HTTP metrics.
-func (s *Middleware) Metrics() func(http.Handler) http.Handler {
-	mdlw := ghmm.New(ghmm.Config{
-		Recorder: metrics.NewRecorder(metrics.Config{}),
-	})
-
-	return func(next http.Handler) http.Handler {
-		return std.Handler("", mdlw, next)
-	}
-}
-
 // MetricsAuth returns middleware that protects metrics endpoints
 // with basic auth.
 func (s *Middleware) MetricsAuth() func(http.Handler) http.Handler {