public function available($id, $start, $end){
        $param = array(
            'start' => date("Y-m-d\TH:i:s\Z", $start),
            'end' => date("Y-m-d\TH:i:s\Z", $end)
        );

        return $this->get('metrics/definitions/'.urlencode($id), $param);
    }