Add connection pool `DEBUG` logs & tracing.

[tomchristie]

Escalated from discussion https://github.com/encode/httpcore/discussions/505

^{Originally posted by inikolaev February 7, 2022} Hello!

Can you advise some way to monitor connection pool, specifically for the AsyncClient?

What I would like to know is the following:

• How many request are awaiting a connection
• How many connections are being used right now (in-flight requests)
• Some statistics on connection open/close

I could potentially get some numbers using existing request and response hook, what I don't know is when request hook is fired - does it fire before connection is acquired or after - it's not very clear to me from the documentation. But this should be easy to check.

Perhaps there could be added hooks into the connection pool or maybe someone from the community does this in a different way somehow already?

Currently the "trace" extension doesn't provide any visibility around the connection pooling.

It would be neat if we had events around this. I'm not sure exactly what set of events we'd want, but essentially we'd like to be able to track information around when connections are added or removed from the pool, and when connections become active or idle.

The first step here would be to determine what set of events we'd want to see, sketch out how the documentation ought to look once we've added those. Once that's done we can tackle the implementation.