refactoring monitor code, support prometheus (#1668)

* refactoring monitor code, support prometheus
* remove vendor

server/dashboard.go

@@ -24,6 +24,7 @@ import (
 	frpNet "github.com/fatedier/frp/utils/net"
 
 	"github.com/gorilla/mux"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
 var (
@@ -38,6 +39,11 @@ func (svr *Service) RunDashboardServer(addr string, port int) (err error) {
 	user, passwd := svr.cfg.DashboardUser, svr.cfg.DashboardPwd
 	router.Use(frpNet.NewHttpAuthMiddleware(user, passwd).Middleware)
 
+	// metrics
+	if svr.cfg.EnablePrometheus {
+		router.Handle("/metrics", promhttp.Handler())
+	}
+
 	// api, see dashboard_api.go
 	router.HandleFunc("/api/serverinfo", svr.ApiServerInfo).Methods("GET")
 	router.HandleFunc("/api/proxy/{type}", svr.ApiProxyByType).Methods("GET")