Label a request with an empty path by the / route in metrics

When both r.URL.Path and r.URL.RawPath are empty, as for the request
line GET http://pixa.example HTTP/1.1, chi routes the request as /, but
routePattern asked Find about the empty path and labelled it unmatched.
It now uses / then, as chi does. After the rebase onto next, the TODO.md
entry is back at the top of Completed Steps.

Model: opus-5-5

internal/middleware/middleware.go

@@ -239,7 +239,7 @@ func (s *Middleware) Metrics() func(http.Handler) http.Handler {
 // "unmatched" when it matches none or is not served by a chi router. The
 // metrics library takes the label before the request is routed, so the
 // router is asked which route matches, with the path it routes on:
-// r.URL.RawPath when set, else r.URL.Path.
+// r.URL.RawPath when set, else r.URL.Path, else /.
 func routePattern(r *http.Request) string {
 	pattern := ""
 
@@ -250,6 +250,10 @@ func routePattern(r *http.Request) string {
 			path = r.URL.Path
 		}
 
+		if path == "" {
+			path = "/"
+		}
+
 		pattern = rctx.Routes.Find(chi.NewRouteContext(), r.Method, path)
 
 		// For the path a subrouter is mounted at, such as /v1 or /v1/, Find