Added logName option

[Lastwisher]

Hello,
Thanks for this package, very simple and useful. I just made one line of insertion to your code.

As explained at resource
https://cloud.google.com/logging/docs/reference/v2/rest/v2/LogEntry

we have a option called "logName" i just added that option. This will allow us to write to diffrent files on log function.

Usage :

$context['stackdriver'] = [
    'logName' => 'projects/[projectIID]/logs/[LogName]'
];

Note ;
normally logname format is

projects/[projectIID]/logs/[LogName]

I can make a new pr that stores project id and fixes short format like

$context['stackdriver'] = [
    'logName' => '[LogName]'
];

Just want to get your opinion before i digg into more.

Thanks.