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

The metrics middleware labelled each request with its path, so every
new path added series to the registry that were never removed, and
/metrics listed the token of every encrypted URL requested. The handler
label is now the pattern of the chi route the request will match, looked
up before routing, or "unmatched" when none does. /v1 and /v1/ ask the
image subrouter, and an empty path is looked up as /, as chi routes it.
TestMetricsRecordsServedRequest expects "unmatched" in place of "/test",
as the owner approved.

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 expired negative cache rows are removed on a schedule (closes
   #244): the row of a failed fetch, which makes pixa answer 502 for that source
   URL for 5 minutes, was deleted only when the same URL was requested again