Add optional pool utilization stats

[shizzard]

I added a way to monitor approximate pool utilization.
TL;DR: When stats collection is enabled, pool manager will collect stats regarding workers utilization. It will report utilization in percentage. Stats reported will show pool utilization in between poolboy:stats/1 calls.

New configuration option added: {stats, boolean()}. Default value: false.

When option set to true, pool manager process will collect time duration workers being checked out. This is done by simply adding yet another duration to a total time worker was busy doing stuff.

When poolboy:stats/1 is called, manager will report collected values (observed period, total workers busy time, original pool size); it will reset observing period time and total workers busy time. Approximate pool utilization calculations are done on caller side: utilization is a float 0.0..1.0 when no overflow allowed, and 0.0..N if overflow is allowed.
NB: when pool is configured with overflow allowed, pool utilization may go over 1.0. This will, obviously, indicate that pool is overflown.

I consider this code to be quite cheap to use in heavily loaded pools. It will allow to monitor pool utilization in more precise manner; pool utilization then may be reported to some monitoring system (e.g. prometheus/grafana, etc.).

Let me know if you like the idea or if this code is good enough to be merged.

[shizzard]

Looks like I will need to support erlang pre-18, since new time API is available starting from erlang 18.0 / erts 7.0.

[shizzard]

@Vagabond hi!
Do you have any described process of accepting/declining pull-requests?
As I can see, last accepted PR was merged in October 2018. No rush here, I just want some transparency.
I want to use this code in my project, but I don't really want to support yet another fork.