frontend: use LISTEN_ADDR, refactor Listen code

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

kubernetes-manifests/frontend.yaml

@@ -42,6 +42,8 @@ spec:
             value: "shippingservice:50051"
           - name: CHECKOUT_SERVICE_ADDR
             value: "checkoutservice:5050"
+          - name: LISTEN_ADDR
+            value: "0.0.0.0"
           resources:
             requests:
               cpu: 100m

src/frontend/main.go

@@ -107,9 +107,17 @@ func main() {
 		Handler:     handler,
 		Propagation: &b3.HTTPFormat{}}
 
+	go func() {
 		log.Infof("starting server on " + addr + ":" + srvPort)
-	go log.Fatal(http.ListenAndServe(addr+":"+secondaryPort, handler))
 		log.Fatal(http.ListenAndServe(addr+":"+srvPort, handler))
+	}()
+
+	go func() {
+		log.Infof("starting healthcheck server on " + addr + ":" + secondaryPort)
+		log.Fatal(http.ListenAndServe(addr+":"+secondaryPort, handler))
+	}()
+
+	select {}
 }
 
 func initTracing(log logrus.FieldLogger) {