public function streaming_request($method, $url, $params=array(), $callback='') {
    if ( ! empty($callback) ) {
      if ( ! is_callable($callback) ) {
        return false;
      }
      $this->config['streaming_callback'] = $callback;
    }
    $this->metrics['start']          = time();
    $this->metrics['interval_start'] = $this->metrics['start'];
    $this->metrics['messages']       = 0;
    $this->metrics['last_messages']  = 0;
    $this->metrics['bytes']          = 0;
    $this->metrics['last_bytes']     = 0;
    $this->config['is_streaming']    = true;
    $this->request($method, $url, $params);
  }