IdoQuery: Allow to check whether joining custom vars is permitted

refs #9029

modules/monitoring/library/Monitoring/Backend/Ido/Query/IdoQuery.php

@@ -795,6 +795,16 @@ abstract class IdoQuery extends DbQuery
         return isset($this->idxAliasTable[$alias]) ? $this->idxAliasTable[$alias] : null;
     }
 
+    /**
+     * Return whether this query allows to join custom variables
+     *
+     * @return  bool
+     */
+    public function allowsCustomVars()
+    {
+        return $this->allowCustomVars;
+    }
+
     /**
      * Return true if the given alias denotes a custom variable
      *