monitoring/lib: Add iterator for iterating over non-pseudo monitoring query columns

refs #9956

modules/monitoring/library/Monitoring/Data/ColumnFilterIterator.php

@@ -0,0 +1,33 @@
+<?php
+/* Icinga Web 2 | (c) 2013-2015 Icinga Development Team | GPLv2+ */
+
+namespace Icinga\Module\Monitoring\Data;
+
+use ArrayIterator;
+use FilterIterator;
+use Zend_Db_Expr;
+
+/**
+ * Iterator over non-pseudo monitoring query columns
+ */
+class ColumnFilterIterator extends FilterIterator
+{
+    /**
+     * Create a new ColumnFilterIterator
+     *
+     * @param array $columns
+     */
+    public function __construct(array $columns)
+    {
+        parent::__construct(new ArrayIterator($columns));
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function accept()
+    {
+        $column = $this->current();
+        return ! ($column instanceof Zend_Db_Expr || $column === '(NULL)');
+    }
+}