private function getRequestHeaders() : array
    {
        // Default Headers Set
        $headers = [
            'Content-Type' => 'application/json',
            'User-Agent'   => sprintf('elasticapm-php/%s', Agent::VERSION),
        ];

        // Add Secret Token to Header
        if ($this->config->get('secretToken') !== null) {
            $headers['Authorization'] = sprintf('Bearer %s', $this->config->get('secretToken'));
        }

        return $headers;
    }