Bring back TRACE logs

[florimondmanca]

I notice the logger.trace() calls we had previously in HTTPX are not present in this package anymore.

I suppose this was done out of simplicity, but I think we should bring them back. They were super useful to debug networking issues, connection management issues, etc.

So here's an issue for trackig purposes. :-)

[lovelydinosaur]

Yeah that's my fault,

I think it was while I was trying to do a bit of refactoring of the HTTP/2 and pulling everything across to httpcore. Is it clear what environment variable naming we ought to be using given that we've got a proper httpx/httpcore split now?

[florimondmanca]

For env vars we could have an HTTPCORE_LOG_LEVEL, and make HTTPX set that when HTTPX_LOG_LEVEL is set too.

[lovelydinosaur]

Sounds reasonable yeah