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/cmd/server/routes_test.go

@@ -159,3 +159,16 @@ func TestBuildMuxObservesRequestsAndServesMetrics(t *testing.T) {
 	require.Equal(t, http.StatusOK, code)
 	assert.Contains(t, body, `path="GET /pending"`)
 }
+
+// hyperguild#185: an authenticated request to an unregistered path reached
+// the inner REST mux, which left Pattern empty, and the metrics label fell
+// back to the raw URL path: one Prometheus series per unique path any token
+// holder sends. It is now labelled with a fixed catch-all.
+func TestBuildMuxLabelsAnUnregisteredPathWithAFixedLabel(t *testing.T) {
+	r := testMux(t)
+	code, _ := do(t, r, "GET", "/zqrandom-185-segment", "Bearer "+routesTok)
+	require.Equal(t, http.StatusNotFound, code)
+	_, body := do(t, r, "GET", "/metrics", "")
+	assert.NotContains(t, body, "zqrandom-185-segment", "the raw path became a metrics label")
+	assert.Contains(t, body, `path="unmatched"`)
+}