refactoring monitor code, support prometheus (#1668)

* refactoring monitor code, support prometheus
* remove vendor

utils/metric/metrics.go

@@ -0,0 +1,34 @@
+// Copyright 2020 fatedier, fatedier@gmail.com
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package metric
+
+// GaugeMetric represents a single numerical value that can arbitrarily go up
+// and down.
+type GaugeMetric interface {
+	Inc()
+	Dec()
+	Set(float64)
+}
+
+// CounterMetric represents a single numerical value that only ever
+// goes up.
+type CounterMetric interface {
+	Inc()
+}
+
+// HistogramMetric counts individual observations.
+type HistogramMetric interface {
+	Observe(float64)
+}

utils/vhost/http.go

@@ -174,7 +174,6 @@ func (rp *HttpReverseProxy) getVhost(domain string, location string) (vr *VhostR
 		}
 		domainSplit = domainSplit[1:]
 	}
-	return
 }
 
 func (rp *HttpReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {

utils/vhost/vhost.go

@@ -116,7 +116,6 @@ func (v *VhostMuxer) getListener(name, path string) (l *Listener, exist bool) {
 		}
 		domainSplit = domainSplit[1:]
 	}
-	return
 }
 
 func (v *VhostMuxer) run() {