Label request metrics by route, not by path (closes #254)

The metrics middleware gave the handler label each request's path, so
every new path added series that were never removed, and /metrics
listed encrypted URL tokens. The label is now the pattern of the chi
route the request matches, or "unmatched" for one that matches none.
The metrics library takes the label before calling the handler, so the
middleware asks the router with chi's Find which route matches.

The existing TestMetricsRecordsServedRequest now fails: it serves a
request outside any router and expects handler="/test", the path.
Changing it needs the owner's approval.

Model: opus-5-5

README.md

@@ -621,7 +621,10 @@ See `configs/config.example.yml` for all options with defaults.
 - **Metrics**: Prometheus, at `/metrics`: generic HTTP request metrics
   (duration, response size, requests in flight) and the Go runtime and process
   metrics; requests are measured and `/metrics` is served only when
-  `metrics.username` and `metrics.password` are set
+  `metrics.username` and `metrics.password` are set. A request's `handler` label
+  is the pattern of the route it matches, such as `/v1/image/*`, never its path.
+  Requests that match no route share the label `unmatched`: those the router
+  answers 404 or 405, and CORS preflights
 - **Logging**: stdlib slog
 
 ## Entrypoints