chore(coderd/httpmw): extract HTTPRoute middleware (#21498)

Extracts part of the prometheus middleware that stores the route
information in the request context into its own middleware. Also adds
request method information to context.

Relates to https://github.com/coder/internal/issues/1214

coderd/coderd.go

@@ -881,6 +881,7 @@ func New(options *Options) *API {
 		loggermw.Logger(api.Logger),
 		singleSlashMW,
 		rolestore.CustomRoleMW,
+		httpmw.HTTPRoute, // NB: prometheusMW depends on this middleware.
 		prometheusMW,
 		// Build-Version is helpful for debugging.
 		func(next http.Handler) http.Handler {