_final callback removed

[lgfa29]

A while back I submitted a PR to fix Winston 3 support, but part of my PR was removed, more specifically that part that implemented the _final callback to flush messages when the transport closes.

I added a comment to the commit but it seems to not have enough visibility.

Would it possible to get the _final callback back in place? It's extremely import in my use case where I am using Open Whisk so I need to flush the messages before my action gets terminated and I lose important log messages.

Thank you.

[lgfa29]

For more information on _final in Winston, check winstonjs/winston#1346 and https://github.com/winstonjs/winston-transport/pull/24/files

[lgfa29]

@respectus @LYHuang any chance someone could you take a look at this?