metrics: add a new router and listener for Prometheus' metrics endpoint

app.go

@@ -68,6 +68,7 @@ const (
 type Config struct {
 	ServerURL                      string
 	Addr                           string
+	MetricsAddr                    string
 	GRPCAddr                       string
 	GRPCAllowInsecure              bool
 	EphemeralNodeInactivityTimeout time.Duration
@@ -406,11 +407,17 @@ func (h *Headscale) ensureUnixSocketIsAbsent() error {
 	return os.Remove(h.cfg.UnixSocket)
 }
 
-func (h *Headscale) createRouter(grpcMux *runtime.ServeMux) *gin.Engine {
+func (h *Headscale) createPrometheusRouter() *gin.Engine {
-	router := gin.Default()
+	promRouter := gin.Default()
 
 	prometheus := ginprometheus.NewPrometheus("gin")
-	prometheus.Use(router)
+	prometheus.Use(promRouter)
+
+	return promRouter
+}
+
+func (h *Headscale) createRouter(grpcMux *runtime.ServeMux) *gin.Engine {
+	router := gin.Default()
 
 	router.GET(
 		"/health",
@@ -622,6 +629,27 @@ func (h *Headscale) Serve() error {
 	log.Info().
 		Msgf("listening and serving HTTP on: %s", h.cfg.Addr)
 
+	promRouter := h.createPrometheusRouter()
+
+	promHttpServer := &http.Server{
+		Addr:         h.cfg.MetricsAddr,
+		Handler:      promRouter,
+		ReadTimeout:  HTTPReadTimeout,
+		WriteTimeout: 0,
+	}
+
+	var promHttpListener net.Listener
+	promHttpListener, err = net.Listen("tcp", h.cfg.MetricsAddr)
+
+	if err != nil {
+		return fmt.Errorf("failed to bind to TCP address: %w", err)
+	}
+
+	errorGroup.Go(func() error { return promHttpServer.Serve(promHttpListener) })
+
+	log.Info().
+		Msgf("listening and serving metrics on: %s", h.cfg.MetricsAddr)
+
 	return errorGroup.Wait()
 }