Add log4j backend

[EdwinKempin]

We want to use Flogger in Gerrit where we currently use slf4j with a log4j backend. We want to continue using log4j as backend so that the switch to Flogger is transparent for existing Gerrit installations and existing log4j configurations continue to work.

[EdwinKempin]

You can assign this issue to me. I'm already looking at ways to implement this.

[cslee00]

Started this for logback: https://github.com/cslee00/digitalascent-flogger-logback, working through how to drop it into Flogger

[ben-manes]

Would there be much harm in having an slf4j backend?

[Bananeweizen]

@ben-manes Do I get it right that you suggest chaining flogger to slf4j to any of the logging frameworks underneath? In a sense that this would be more flexible than the suggest log4j-only implementation? If so, I'm +1. :)

[cslee00]

Clunky to chain together two logging APIs (flogger, slf4j) - there are overlaps in responsibilities as they both provide a facade on back-end logging implemementations. Cleanest from an implementation & performance perspective to use back-end loggers directly.

[EdwinKempin]

This got merged now:
#23

[davido]

What about log4j2 backend? There are these CLs pending for review for Gerrit: [1] and many other distros, e.g. ElasticSearch (that Gerrit currently depends on) already migrated to log4j2.

[1] https://gerrit-review.googlesource.com/#/q/topic:update-log4j-to-log4j2

[EdwinKempin]

I think writing a backend for log4j2 would be straight-forward now that we have one for log4j, but it's nothing I'm planning to look at.

[paladox]

Though log4j2 syntax is significantly different to log4j.