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

TODO.md

@@ -30,6 +30,15 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-15 the request metrics are labelled by route, not by path (closes
+  #254): their `handler` label was each request's path, so every new path, a 404
+  or an encrypted URL, added series to the Prometheus registry that were never
+  removed, and `/metrics` listed every encrypted URL's token. The label is now
+  the pattern of the chi route the request matches, such as `/v1/image/*`, or
+  `unmatched` for a request that matches none: one the router answers 404 or
+  405, or a CORS preflight. The metrics library takes the label before the
+  request is routed, so the metrics middleware asks the router which route
+  matches.
 - 2026-10-15 a cached variant is served while its source URL is in the negative
   cache (closes #245): `Get` checked the negative cache before the variant
   cache, so once a fetch of the source failed, as one does when another variant
@@ -40,15 +49,6 @@ P2: security: per-origin rate limiting
   not in the cache is made from the cached source when there is one. `README.md`
   says so under `/v1/image/`. A refusal from the negative cache now counts as a
   miss, as any other request that misses the variant cache does.
-- 2026-10-15 the request metrics are labelled by route, not by path (closes
-  #254): their `handler` label was each request's path, so every new path, a 404
-  or an encrypted URL, added series to the Prometheus registry that were never
-  removed, and `/metrics` listed every encrypted URL's token. The label is now
-  the pattern of the chi route the request matches, such as `/v1/image/*`, or
-  `unmatched` for a request that matches none: one the router answers 404 or
-  405, or a CORS preflight. The metrics library takes the label before the
-  request is routed, so the metrics middleware asks the router which route
-  matches.
 - 2026-10-15 the host of a `/v1/image/` URL is lower-cased, and one with a port
   needs a signature (closes #246): with `cdn.example.com` allowlisted,
   `CDN.Example.com` and `cdn.example.com:8443` passed the allowlist too, and

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