DataView\Eventgrid: Allow to filter for custom variables

refs #9009

modules/monitoring/library/Monitoring/DataView/Eventgrid.php

@@ -5,6 +5,18 @@ namespace Icinga\Module\Monitoring\DataView;
 
 class Eventgrid extends DataView
 {
+    /**
+     * {@inheritdoc}
+     */
+    public function isValidFilterTarget($column)
+    {
+        if ($column[0] === '_' && preg_match('/^_(?:host|service)_/', $column)) {
+            return true;
+        }
+
+        return parent::isValidFilterTarget($column);
+    }
+
     /**
      * Retrieve columns provided by this view
      *