Support capturing metrics in forked processes

[liaden]

I opened an issue at yabeda-rb/yabeda-puma-plugin#18 (with some code in it) for the puma perspective, but it may be more appropriate here since there are other forking configurations in ruby such as sidekiq swarm and unicorn.

I can open an PR for Puma if it is a good fit here.

[Envek]

Probably it should be done in Yabeda itself, but honestly I have no idea how to properly do that synchronized calling of collect blocks between processes. Actually, many use cases of this, here is the issue to collect them: yabeda-rb/yabeda#32