Decrease log level for unsupported statistics keys

Interfaces may have a lot of statistics keys which are unexpected by this
exporter. It does not break operation, but causes annoying spam of Error messages in the log.
Let's set it to Debug.

pkg/ovs_exporter/ovs_exporter.go

@@ -1120,7 +1120,7 @@ func (e *Exporter) GatherMetrics() {
 						intf.UUID,
 					))
 				default:
-					level.Error(e.logger).Log(
+					level.Debug(e.logger).Log(
 						"msg", "detected malformed interface statistics",
 						"system_id", e.Client.System.ID,
 						"key", key,