Label request metrics with standard HTTP methods only (closes #274)

The metrics middleware labelled each request with its method as sent,
and Go's HTTP server accepts any method name that is a valid token, so
each made-up method a client sent added series that were never removed.
The method label is now the request's method only for the nine standard
methods, and "other" for any other. The middleware hands the metrics
library a copy of the request carrying the label and serves the next
handler the original, so handlers still see the method the client sent.

Model: opus-5-5

README.md

@@ -685,7 +685,9 @@ See `configs/config.example.yml` for all options with defaults.
   `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
+  answers 404 or 405, and CORS preflights. A request's `method` label is its
+  method when that is `GET`, `HEAD`, `POST`, `PUT`, `PATCH`, `DELETE`,
+  `OPTIONS`, `CONNECT` or `TRACE`, and `other` for any other method
 - **Logging**: stdlib slog
 
 ## Entrypoints