fix(metrics): label an unmatched request "unmatched", never its raw path (#185)

An authenticated REST request to an unregistered path reached the inner
mux, which left Pattern empty, and metrics.Middleware fell back to the
raw URL path: any token holder could create one Prometheus series per
unique path. The fallback is now the fixed label "unmatched"; real routes
keep their pattern. Tests: buildMux with an authenticated random path,
and the middleware unit test; the timing test now routes through a
ServeMux as production does.

Fixes #185.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ingestion/internal/metrics/metrics.go

@@ -83,8 +83,9 @@ func (r *Registry) Observe(path, status string, d time.Duration) {
 
 // Middleware wraps next, observing every request's duration + status.
 // The metric label `path` uses the request's Pattern (Go 1.22+ ServeMux),
-// falling back to the URL path if no Pattern is set. Pattern keeps
-// cardinality bounded (one series per route, not one per unique URL).
+// or the fixed label "unmatched" if no Pattern is set: falling back to the
+// raw URL path gave any caller one series per unique path (hyperguild#185).
+// Pattern keeps cardinality bounded (one series per route).
 func (r *Registry) Middleware(next http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
 		rec := &statusRecorder{ResponseWriter: w, code: http.StatusOK}
@@ -92,7 +93,7 @@ func (r *Registry) Middleware(next http.Handler) http.Handler {
 		next.ServeHTTP(rec, req)
 		path := req.Pattern
 		if path == "" {
-			path = req.URL.Path
+			path = "unmatched"
 		}
 		r.Observe(path, statusClass(rec.code), time.Since(start))
 	})