Geo secondary proxying: serve assets locally

As Geo sites are required to be on the same version as the primary,
the assets should be the same, and at the same path, therefore
we can speed this up by serving them from the local secondary
when proxying is in use.

Changelog: changed
EE: true

workhorse/internal/upstream/routes.go

@@ -397,6 +397,16 @@ func configureRoutes(u *upstream) {
 		// Internal API routes
 		u.route("", "^/api/v4/internal", defaultUpstream),
 
+		u.route(
+			"", `^/assets/`,
+			static.ServeExisting(
+				u.URLPrefix,
+				staticpages.CacheExpireMax,
+				assetsNotFoundHandler,
+			),
+			withoutTracing(), // Tracing on assets is very noisy
+		),
+
 		// Don't define a catch-all route. If a route does not match, then we know
 		// the request should be proxied.
 	}