add a custom loger filer

services:
  my_logger:
    class: Monolog\Logger
    arguments: [my_info]
    calls:
        - [pushHandler, [@my_log_handler]]

  my_log_handler:
    class: Monolog\Handler\StreamHandler
    arguments: [%kernel.root_dir%/logs/my_info.log, 100]

Usage (in Controller, for example):

$this->get('my_logger')->info('info message');