Prometheus endpoint (#5256)

* Add prometheus collector and route

* dep ensure -add github.com/prometheus/client_golang/prometheus

* dep ensure -update github.com/golang/protobuf

* add metrics to reserved usernames

* add comment head in metrics package

* fix style imports

* add metrics settings

* add bearer token check

* mapping metrics configs

* fix lint

* update config cheat sheet

* update conf sample, typo fix

routers/metrics.go

@@ -0,0 +1,30 @@
+// Copyright 2018 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package routers
+
+import (
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+
+	"code.gitea.io/gitea/modules/context"
+	"code.gitea.io/gitea/modules/setting"
+)
+
+// Metrics validate auth token and render prometheus metrics
+func Metrics(ctx *context.Context) {
+	if setting.Metrics.Token == "" {
+		promhttp.Handler().ServeHTTP(ctx.Resp, ctx.Req.Request)
+		return
+	}
+	header := ctx.Header().Get("Authorization")
+	if header == "" {
+		ctx.Error(401)
+		return
+	}
+	if header != "Bearer "+setting.Metrics.Token {
+		ctx.Error(401)
+		return
+	}
+	promhttp.Handler().ServeHTTP(ctx.Resp, ctx.Req.Request)
+}