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

internal/middleware/middleware.go

@@ -220,7 +220,10 @@ func (s *Middleware) CORS() func(http.Handler) http.Handler {
 // pattern of the chi route a request matches, such as /v1/image/*, or
 // "unmatched" for a request that matches none, so the routes fix how many
 // values the label has. It is never the path, with which any client could
-// add series to the registry without limit.
+// add series to the registry without limit. Its method label is the
+// request's method when that is a standard one, such as GET, and "other"
+// for any other: Go's HTTP server accepts any method name, so a label per
+// method would let a client add series the same way.
 func (s *Middleware) Metrics() func(http.Handler) http.Handler {
 	mdlw := ghmm.New(ghmm.Config{
 		Recorder: metrics.NewRecorder(metrics.Config{
@@ -230,7 +233,19 @@ func (s *Middleware) Metrics() func(http.Handler) http.Handler {
 
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-			std.Handler(routePattern(r), mdlw, next).ServeHTTP(w, r)
+			// The metrics library takes the method label from the request
+			// it is handed, so it is handed a shallow copy of r whose
+			// method is the label. next is served r itself, through the
+			// response writer the library wraps to see the status and size.
+			labelled := r.WithContext(r.Context())
+			labelled.Method = methodLabel(r.Method)
+
+			serveNext := http.HandlerFunc(
+				func(wrapped http.ResponseWriter, _ *http.Request) {
+					next.ServeHTTP(wrapped, r)
+				})
+
+			std.Handler(routePattern(r), mdlw, serveNext).ServeHTTP(w, labelled)
 		})
 	}
 }
@@ -275,6 +290,20 @@ func routePattern(r *http.Request) string {
 	return pattern
 }
 
+// methodLabel returns method when it is one of the standard methods net/http
+// names: GET, HEAD, POST, PUT, PATCH, DELETE, OPTIONS, CONNECT and TRACE.
+// For any other method it returns "other".
+func methodLabel(method string) string {
+	switch method {
+	case http.MethodGet, http.MethodHead, http.MethodPost, http.MethodPut,
+		http.MethodPatch, http.MethodDelete, http.MethodOptions,
+		http.MethodConnect, http.MethodTrace:
+		return method
+	default:
+		return "other"
+	}
+}
+
 // MetricsAuth returns a basic auth middleware for the metrics endpoint.
 func (s *Middleware) MetricsAuth() func(http.Handler) http.Handler {
 	return basicauth.New(