feat: add prometheus metrics (#342)

server/api_server.go

@@ -5,6 +5,7 @@ import (
 	"net/http"
 
 	"github.com/gorilla/mux"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/sirupsen/logrus"
 )
 
@@ -15,6 +16,8 @@ func StartApiServer(apiBinding string) {
 
 	apiRoutes.Path("/vars").Handler(expvar.Handler())
 
+	apiRoutes.Path("/metrics").Handler(promhttp.Handler())
+
 	go func() {
 		logrus.WithError(
 			http.ListenAndServe(apiBinding, apiRoutes)).Error("API server failed")

server/connector.go

@@ -26,10 +26,11 @@ const (
 var noDeadline time.Time
 
 type ConnectorMetrics struct {
-	Errors            metrics.Counter
-	BytesTransmitted  metrics.Counter
-	Connections       metrics.Counter
-	ActiveConnections metrics.Gauge
+	Errors              metrics.Counter
+	BytesTransmitted    metrics.Counter
+	ConnectionsFrontend metrics.Counter
+	ConnectionsBackend  metrics.Counter
+	ActiveConnections   metrics.Gauge
 }
 
 func NewConnector(metrics *ConnectorMetrics, sendProxyProto bool, receiveProxyProto bool, trustedProxyNets []*net.IPNet) *Connector {
@@ -158,7 +159,7 @@ func (c *Connector) acceptConnections(ctx context.Context, ln net.Listener, conn
 }
 
 func (c *Connector) HandleConnection(ctx context.Context, frontendConn net.Conn) {
-	c.metrics.Connections.With("side", "frontend").Add(1)
+	c.metrics.ConnectionsFrontend.Add(1)
 	//noinspection GoUnhandledErrorResult
 	defer frontendConn.Close()
 
@@ -276,7 +277,7 @@ func (c *Connector) findAndConnectBackend(ctx context.Context, frontendConn net.
 		return
 	}
 
-	c.metrics.Connections.With("side", "backend", "host", resolvedHost).Add(1)
+	c.metrics.ConnectionsBackend.With("host", resolvedHost).Add(1)
 
 	c.metrics.ActiveConnections.Set(float64(
 		atomic.AddInt32(&c.activeConnections, 1)))