emails/ui/setup/setups/98_log.php

<?php
use Psr\Container\ContainerInterface;

return [
    Monolog\Handler\DeduplicationHandler::class => function(ContainerInterface $container) {
        return new Monolog\Handler\DeduplicationHandler($container->get(Monolog\Handler\RotatingFileHandler::class));
    },
    Monolog\Handler\RotatingFileHandler::class => function(ContainerInterface $container) {
        $handler = new Monolog\Handler\RotatingFileHandler($container->get('log_file'));
        $handler->setFormatter($container->get(Monolog\Formatter\SyslogFormatter::class));
        return $handler;
    },
    Psr\Log\LoggerInterface::class => function(ContainerInterface $container) {
        $logger = new Monolog\Logger('file_logger');
        $logger->pushHandler($container->get(Monolog\Handler\DeduplicationHandler::class));
        //$logger->pushHandler($container->get(Monolog\Handler\RotatingFileHandler::class));
        $logger->pushProcessor($container->get(Monolog\Processor\PsrLogMessageProcessor::class));
        $logger->pushProcessor($container->get(Monolog\Processor\IntrospectionProcessor::class));
        $logger->pushProcessor($container->get(Monolog\Processor\MemoryUsageProcessor::class));
        return $logger;
    }
];