<?php
return [
    'components' => [
        'log' => function() {
            $target = new \craft\log\MonologTarget(['name' => 'myTarget']);
            $target->getLogger()->setHandlers([
                (new \Monolog\Handler\StreamHandler('php://stderr', \Monolog\Logger::ERROR))
                    ->setFormatter($target->getFormatter()),
            ]);
        
            $dispatcher = new \craft\log\Dispatcher();
            $dispatcher->targets[] = $target;
        
            return $dispatcher;
        },
    ],
];

<?php
// After
return [
    'components' => [
        'log' => [
            'targets' => [
                [
                    'class' => \craft\log\MonologTarget::class,
                    'name' => 'myTarget',
                    'logger' => fn(\craft\log\MonologTarget $target) => $target
                        ->getLogger()
                        ->setHandlers([
                            (new \Monolog\Handler\StreamHandler('php://stdout'))
                                ->setFormatter($target->getFormatter()),
                        ]
                    ),
                ]
            ]
        ],
    ],
];

<?php
return [
    'components' => [
        'log' => [
            'monologTargetConfig' => [
                'logContext' => false,
                'logger' => fn(\craft\log\MonologTarget $target) => $target
                    ->getLogger()
                    ->pushHandler(
                        (new \Monolog\Handler\StreamHandler('php://stdout'))
                            ->setFormatter($target->getFormatter())
                    ),
            ],
        ],
    ],
];