Logging Guzzle with monolog

$monologHandler = new Monolog\Logger(
    'Logger',
    array(
        new Monolog\Handler\StreamHandler('/tmp/log.txt', Monolog\Logger::DEBUG)
    )
);

$stack = GuzzleHttp\HandlerStack::create();
$stack->push(
    GuzzleHttp\Middleware::log(
        $monologHandler,
        new GuzzleHttp\MessageFormatter('{req_body} - {res_body}')
    )
);
$guzzleClient = new \GuzzleHttp\Client(
    array('handler' => $stack)
);