[feature] Introduce new %{callpath} format keyword

[nplanel]

%{callpath} aim to print the callers stack as a call path.

Output exmaples :
main.a.b.c
main.a.b.c...c

"..." stand for recursive call

[op]

@nplanel Nifty! Feels like you'd want to limit the stack trace in most applications though.

[nplanel]

Thanks,
Yes I think it would be good to limit it by an argument, like %{callpath:5} last 5 callers

I was thinking about introduce another patch like to print extra information based on the level, ex:
%{pid:DEBUG} would be print if the module level is set to DEBUG even if the format is specified in an log.Info().
The Idea is to be more verbose on specific message, when you are in production and you would like more info when debugging in production.

What do think about ?

[op]

@nplanel The idea tickles my interest but maybe it would be nicer to have separate formatters per log level? I hope that should be possible to create by combining the existing infrastructure.

[nplanel]

@op After re-thinking I think using different formatters (on the app side) is the right way to do so.