Metrics: Add direct listen (#4409)

Author: Fangliding

app/metrics/config.pb.go

@@ -10,4 +10,5 @@ option java_multiple_files = true;
 message Config {
   // Tag of the outbound handler that handles metrics http connections.
   string tag = 1;
+  string listen = 2;
 }

app/metrics/config.proto

@@ -24,12 +24,15 @@ type MetricsHandler struct {
 	statsManager feature_stats.Manager
 	observatory  extension.Observatory
 	tag          string
+	listen       string
+	tcpListener  net.Listener
 }
 
 // NewMetricsHandler creates a new MetricsHandler based on the given config.
 func NewMetricsHandler(ctx context.Context, config *Config) (*MetricsHandler, error) {
 	c := &MetricsHandler{
-		tag: config.Tag,
+		tag:    config.Tag,
+		listen: config.Listen,
 	}
 	common.Must(core.RequireFeatures(ctx, func(om outbound.Manager, sm feature_stats.Manager) {
 		c.statsManager = sm
@@ -87,6 +90,23 @@ func (p *MetricsHandler) Type() interface{} {
 }
 
 func (p *MetricsHandler) Start() error {
+
+	// direct listen a port if listen is set
+	if p.listen != "" {
+		TCPlistener, err := net.Listen("tcp", p.listen)
+		if err != nil {
+			return err
+		}
+		p.tcpListener = TCPlistener
+		errors.LogInfo(context.Background(), "Metrics server listening on ", p.listen)
+
+		go func() {
+			if err := http.Serve(TCPlistener, http.DefaultServeMux); err != nil {
+				errors.LogErrorInner(context.Background(), err, "failed to start metrics server")
+			}
+		}()
+	}
+
 	listener := &OutboundListener{
 		buffer: make(chan net.Conn, 4),
 		done:   done.New(),

app/metrics/metrics.go

@@ -6,15 +6,21 @@ import (
 )
 
 type MetricsConfig struct {
-	Tag string `json:"tag"`
+	Tag    string `json:"tag"`
+	Listen string `json:"listen"`
 }
 
 func (c *MetricsConfig) Build() (*metrics.Config, error) {
+	if c.Listen == "" && c.Tag == "" {
+		return nil, errors.New("Metrics must have a tag or listen address.")
+	}
+	// If the tag is empty but have "listen" set a default "Metrics" for compatibility.
 	if c.Tag == "" {
-		return nil, errors.New("metrics tag can't be empty.")
+		c.Tag = "Metrics"
 	}
 
 	return &metrics.Config{
-		Tag: c.Tag,
+		Tag:    c.Tag,
+		Listen: c.Listen,
 	}, nil
 }