protected function addRetrySubscriber(GuzzleClient $guzzle)
    {
        // Build retry subscriber
        $retry = new RetrySubscriber([
            'filter' => RetrySubscriber::createStatusFilter($this->retryOn),
            'delay'  => function ($number, $event) {
                return $this->retryDelay;
            },
            'max' => $this->retry,
        ]);

        // add the retry emitter
        $guzzle->getEmitter()->attach($retry);

        return $guzzle;
    }