protected function getHeaders($maxAttempts, $remainingAttempts, $retryAfter = null)
    {
        $headers = [
            'X-RateLimit-Limit' => $maxAttempts,
            'X-RateLimit-Remaining' => $remainingAttempts,
        ];

        if (! is_null($retryAfter)) {
            $headers['Retry-After'] = $retryAfter;
            $headers['X-RateLimit-Reset'] = $this->availableAt($retryAfter);
        }

        return $headers;
    }