refactoring monitor code, support prometheus (#1668)

* refactoring monitor code, support prometheus
* remove vendor

server/metrics/metrics.go

@@ -0,0 +1,37 @@
+package metrics
+
+import (
+	"sync"
+)
+
+type ServerMetrics interface {
+	NewClient()
+	CloseClient()
+	NewProxy(name string, proxyType string)
+	CloseProxy(name string, proxyType string)
+	OpenConnection(name string, proxyType string)
+	CloseConnection(name string, proxyType string)
+	AddTrafficIn(name string, proxyType string, trafficBytes int64)
+	AddTrafficOut(name string, proxyType string, trafficBytes int64)
+}
+
+var Server ServerMetrics = noopServerMetrics{}
+
+var registerMetrics sync.Once
+
+func Register(m ServerMetrics) {
+	registerMetrics.Do(func() {
+		Server = m
+	})
+}
+
+type noopServerMetrics struct{}
+
+func (noopServerMetrics) NewClient()                                                      {}
+func (noopServerMetrics) CloseClient()                                                    {}
+func (noopServerMetrics) NewProxy(name string, proxyType string)                          {}
+func (noopServerMetrics) CloseProxy(name string, proxyType string)                        {}
+func (noopServerMetrics) OpenConnection(name string, proxyType string)                    {}
+func (noopServerMetrics) CloseConnection(name string, proxyType string)                   {}
+func (noopServerMetrics) AddTrafficIn(name string, proxyType string, trafficBytes int64)  {}
+func (noopServerMetrics) AddTrafficOut(name string, proxyType string, trafficBytes int64) {}