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.

Model: opus-5-5

TODO.md

@@ -41,6 +41,15 @@ P2: security: per-origin rate limiting
   not in `allowlist.IsAllowed`, which also matches `referer_blocklist`, where a
   `Referer` is blocked whatever its port. A URL is signed over its host as it
   writes it, in lower case, with any IPv6 brackets, `:` and port kept.
+- 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 `:` in the source path or query no longer moves where the path
   ends and the query begins (closes #247): the signed data and the cache key
   joined host, path and query with `:` and no escaping, so a URL signed for