plugin/metrics: Added New func (#1309)

If external plugins wanted to extend metrics there was no way since
zoneNames couldn't be initialized. Now plugins can call New to get an
instance of Metrics that they can extend.

plugin/metrics/metrics_test.go

@@ -13,7 +13,7 @@ import (
 )
 
 func TestMetrics(t *testing.T) {
-	met := &Metrics{Addr: "localhost:0", zoneMap: make(map[string]bool)}
+	met := New("localhost:0")
 	if err := met.OnStartup(); err != nil {
 		t.Fatalf("Failed to start metrics handler: %s", err)
 	}