fix self-monitoring (istio#2140)

Automatic merge from submit-queue.

fix self-monitoring

The current self-monitoring is broken, nothing can be fetched
for /metrics nor /version. This is because ListenAndServe() *creates* a new
net.Listener and then serve, while current monitoring.go also creates
a net.Listener instance beforehand, so ListenAndServe() always fails
with "address already in-use" error.

When a net.Listener is given, use "Serve" instead. Also "Close"
can close the listener, the `monitor` does not have to keep the listener
at all. That's all taken care of already.

**What this PR does / why we need it**:

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

**Special notes for your reviewer**:

**Release note**:

```release-note
none
```

mixer/pkg/server/monitoring.go

@@ -26,7 +26,6 @@ import (
 )
 
 type monitor struct {
-	listener         net.Listener
 	monitoringServer *http.Server
 	shutdown         chan struct{}
 }
@@ -42,8 +41,9 @@ func startMonitor(port uint16) (*monitor, error) {
 	}
 
 	// get the network stuff setup
+	var listener net.Listener
 	var err error
-	if m.listener, err = net.Listen("tcp", fmt.Sprintf(":%d", port)); err != nil {
+	if listener, err = net.Listen("tcp", fmt.Sprintf(":%d", port)); err != nil {
 		return nil, fmt.Errorf("unable to listen on socket: %v", err)
 	}
 
@@ -60,19 +60,18 @@ func startMonitor(port uint16) (*monitor, error) {
 	})
 
 	m.monitoringServer = &http.Server{
-		Addr:    fmt.Sprintf(":%d", port),
 		Handler: mux,
 	}
 
 	go func() {
 		m.shutdown <- struct{}{}
-		_ = m.monitoringServer.ListenAndServe()
+		_ = m.monitoringServer.Serve(listener)
 		m.shutdown <- struct{}{}
 	}()
 
-	// This is here to work around (mostly) a race condition in the ListenAndServe
+	// This is here to work around (mostly) a race condition in the Serve
 	// function. If the Close method is called before or during the execution of
-	// ListenAndServe, the call may be ignored and ListenAndServe never returns.
+	// Serve, the call may be ignored and Serve never returns.
 	<-m.shutdown
 
 	return m, nil
@@ -81,6 +80,5 @@ func startMonitor(port uint16) (*monitor, error) {
 func (m *monitor) Close() error {
 	_ = m.monitoringServer.Close()
 	<-m.shutdown
-	_ = m.listener.Close()
 	return nil
 }