explicitely initialize $virtualserver_label_values

otherwise multiple virtualservers get appended and the labels are wrong

src/metrics.php

@@ -40,6 +40,8 @@ try {
             return is_object($info_raw) ? $info_raw->toString() : $info_raw;
         }, $virtualserver->getInfo());
 
+        // initialize with an empty array
+        $virtualserver_label_values = [];
         // fill $virtualserver_label_values
         foreach($virtualserver_label_keys as $virtualserver_label_key) {
             $virtualserver_label_values[] = $infos[$virtualserver_label_key];