How to output log to StackDriver logging in fuelPHP

I deployed app is created by fuelPHP on GKE.

GKE Cluster is enabled Stackdriver Kubernetes Engine Monitoring. https://cloud.google.com/monitoring/kubernetes-engine

But fuelPHP's Log::xxxx() is not output to stackdriver logging. Then I modified config.php following.

//'log_path' =>APPPATH. 'logs',
'log_path' => '/dev/stdout',

It's not work.(error occur '/dev/stdout does not exist').

How to output logs to StackDriver logging?

I override core/Log class as app/Log.

app/classes/log.php

<?php

class Log extends \Fuel\Core\Log
{
    public static function initialize()
    {

        //$stream = new \Monolog\Handler\StreamHandler($filename, \Monolog\Logger::DEBUG);
        $stream = new \Monolog\Handler\StreamHandler('php://stdout', \Monolog\Logger::DEBUG);
        $formatter = new \Monolog\Formatter\LineFormatter("%level_name% - %datetime% --> %message%".PHP_EOL, "Y-m-d H:i:s");
        $stream->setFormatter($formatter);
        static::$monolog->pushHandler($stream);
    }
}

and in app/bootstrap.php. Add as in the example

// Example: 'Log' => APPPATH.'classes/log.php',