Fix issue greenpau#3

Added interface name label to each metric related to the interface

pkg/ovs_exporter/ovs_exporter.go

@@ -331,7 +331,7 @@ type Exporter struct {
 
 type Options struct {
 	Timeout int
-	Logger log.Logger
+	Logger  log.Logger
 }
 
 // NewLogger returns an instance of logger.