Laravel Lumen v5.4 Monolog Logger (Graylog Gelf logger) integration

app.php

<?php
//bootstrap/app.php:
//...
$app->configureMonologUsing(function ($monolog) use ($app) {

      $publisher = new \Gelf\Publisher(new \Gelf\Transport\HttpTransport(env('GRAYLOG_HOST'), env('GRAYLOG_PORT'), env('GRAYLOG_PATH')));

      $failureHandler = new \Monolog\Handler\WhatFailureGroupHandler([new \Monolog\Handler\GelfHandler($publisher)]);

      $monolog->pushHandler($failureHandler);

    //fixes error: A facade root has not been set
    return $monolog;
});

//...
return $app;