Test that metrics label requests for /v1 and /v1/ unmatched

No route answers /v1 or /v1/, the path the image subrouter is mounted
at, and the router answers both with 404, yet the metrics middleware
labels them "/v1" and "/v1/". The test now sends both and expects them
counted under "unmatched". It fails.

Model: opus-5-5

internal/middleware/middleware_internal_test.go

@@ -226,7 +226,8 @@ func TestMetricsRecordsServedRequest(t *testing.T) {
 // TestMetricsLabelsRequestsByRoute checks that the metrics middleware labels
 // each request with the pattern of the route that answers it, never with its
 // path: two paths one route answers share that route's pattern, and requests
-// no route answers, a 404 or a 405, share "unmatched". With a label per path,
+// no route answers, a 404 or a 405, share "unmatched", among them requests for
+// the path the subrouter is mounted at, /v1 and /v1/. With a label per path,
 // any client could add series to the registry without limit. The middleware
 // registers with a registry of its own here, as another test in this package
 // registers with Prometheus' default registry, which can be done only once.
@@ -254,6 +255,8 @@ func TestMetricsLabelsRequestsByRoute(t *testing.T) {
 		{http.MethodGet, "/v1/image/b.example.com/two.jpg/1x1.png"},
 		{http.MethodGet, "/no-such-route"},
 		{http.MethodGet, "/v1/no-such-route"},
+		{http.MethodGet, "/v1"},
+		{http.MethodGet, "/v1/"},
 		{http.MethodPost, "/v1/image/a.example.com/one.jpg/1x1.png"},
 	} {
 		router.ServeHTTP(httptest.NewRecorder(), httptest.NewRequestWithContext(
@@ -263,7 +266,7 @@ func TestMetricsLabelsRequestsByRoute(t *testing.T) {
 	got := requestCountsByHandler(t, registry)
 	t.Logf("requests counted by handler label: %v", got)
 
-	want := map[string]uint64{"/v1/image/*": 2, "unmatched": 3}
+	want := map[string]uint64{"/v1/image/*": 2, "unmatched": 5}
 	if !maps.Equal(got, want) {
 		t.Errorf("requests counted by handler label = %v, want %v", got, want)
 	}