i have done that like this.
I wrote a hanlder base upon the filehandler then i change the config to add this handler
1) first the handler
i copied the system/log/filehandler.php to system/log/stderrhandler.php
I removed everything concerning the file.
i added "echo $msg" inside the function handle.
2) the config
inside the app/Config/Logger.php i added my new handler
Codeingiter\Log\Handlers\stderrhandler =>[handles ...] (just copy it from the chromehandler