Updated metrics.go to add listener add unit tests

Issue: COSI-65

pkg/metrics/metrics.go

@@ -1,6 +1,7 @@
 package metrics
 
 import (
+	"net"
 	"net/http"
 
 	c "github.com/scality/cosi-driver/pkg/constants"
@@ -25,17 +26,24 @@ func init() {
 }
 
 func StartMetricsServer(addr string) (*http.Server, error) {
+	listener, err := net.Listen("tcp", addr)
+	if err != nil {
+		return nil, err
+	}
+	return StartMetricsServerWithListener(listener)
+}
+
+func StartMetricsServerWithListener(listener net.Listener) (*http.Server, error) {
 	mux := http.NewServeMux()
 	mux.Handle(c.MetricsPath, promhttp.Handler())
 
 	srv := &http.Server{
-		Addr:    addr,
 		Handler: mux,
 	}
 
 	go func() {
-		klog.InfoS("Starting Prometheus metrics server", "address", addr)
-		if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+		klog.InfoS("Starting Prometheus metrics server", "address", listener.Addr().String())
+		if err := srv.Serve(listener); err != nil && err != http.ErrServerClosed {
 			klog.ErrorS(err, "Failed to start metrics server")
 		}
 	}()