Label requests for a subrouter's own path unmatched in metrics

For /v1 and /v1/, the path the image subrouter is mounted at, chi's
Find answers that path as the pattern without asking the subrouter,
which chi hands the request as / and which has no route there, so the
router answers 404. routePattern now asks the subrouter mounted there
whether it has a route for /, and labels the request "unmatched" when
it has none.

Model: opus-5-5

internal/middleware/middleware.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"net/netip"
 	"regexp"
+	"strings"
 	"time"
 
 	basicauth "github.com/99designs/basicauth-go"
@@ -250,6 +251,17 @@ func routePattern(r *http.Request) string {
 		}
 
 		pattern = rctx.Routes.Find(chi.NewRouteContext(), r.Method, path)
+
+		// For the path a subrouter is mounted at, such as /v1 or /v1/, Find
+		// answers that path without asking the subrouter, which chi hands
+		// the request as /.
+		for _, route := range rctx.Routes.Routes() {
+			if route.SubRoutes != nil &&
+				route.Pattern == strings.TrimSuffix(pattern, "/")+"/*" &&
+				!route.SubRoutes.Match(chi.NewRouteContext(), r.Method, "/") {
+				pattern = ""
+			}
+		}
 	}
 
 	if pattern == "" {