config.yml

# ...

monolog:
    channels: ['user_action_db', 'customer_email_db']
    handlers:
        user_action_db:
            channels: ['user_action_db']
            type: service
            id: monolog.db_handler
        customer_email_db:
            channels: ['customer_email_db']
            type: service
            id: monolog.db_handler

DefaultController.php

<?php
class DefaultController extends Controller
{
    /**
     * @Route("/", name="homepage")
     */
    public function indexAction(Request $request)
    {
        $this->get('monolog.logger.user_action_db')->info('test');
        
        // replace this example code with whatever you need
        return $this->render('default/index.html.twig', [
            'base_dir' => realpath($this->getParameter('kernel.project_dir')).DIRECTORY_SEPARATOR,
        ]);
    }
}

MonologHandler.php

<?php
namespace AppBundle\Util;

use Monolog\Handler\AbstractProcessingHandler;

class MonologHandler extends AbstractProcessingHandler
{
    protected function write(array $record)
    {
        // TODO: Implement write() method.
    }
}

services.yml

# ...

services:
    # ...
    
    monolog.db_handler:
        class: AppBundle\Util\MonologHandler