Handle /derp/latency-check (#2227)

According to 15fc6cd966
the routes `/derp/probe` and `/derp/latency-check` are the same and
different versions of the tailscale client use one or the other
endpoint.

Also handle /derp/latency-check

Fixes: #2211

hscontrol/app.go

@@ -460,6 +460,7 @@ func (h *Headscale) createRouter(grpcMux *grpcRuntime.ServeMux) *mux.Router {
 	if h.cfg.DERP.ServerEnabled {
 		router.HandleFunc("/derp", h.DERPServer.DERPHandler)
 		router.HandleFunc("/derp/probe", derpServer.DERPProbeHandler)
+		router.HandleFunc("/derp/latency-check", derpServer.DERPProbeHandler)
 		router.HandleFunc("/bootstrap-dns", derpServer.DERPBootstrapDNSHandler(h.DERPMap))
 	}
 

hscontrol/metrics.go

@@ -78,7 +78,7 @@ func prometheusMiddleware(next http.Handler) http.Handler {
 
 		// Ignore streaming and noise sessions
 		// it has its own router further down.
-		if path == "/ts2021" || path == "/machine/map" || path == "/derp" || path == "/derp/probe" || path == "/bootstrap-dns" {
+		if path == "/ts2021" || path == "/machine/map" || path == "/derp" || path == "/derp/probe" || path == "/derp/latency-check" || path == "/bootstrap-dns" {
 			next.ServeHTTP(w, r)
 			return
 		}