Add 'trace' log level

[tomchristie]

Adds a new log level, TRACE, which is a level below DEBUG.

Here's why I like this...

INFO Output for stuff such as Process startup, Database connected, Access logs.
Typically suitable for production.

DEBUG Output for stuff such as SQL on database accesses, Outgoing HTTP requests, Cache lookups, SMTP requests, Background tasks.
Really useful during development, without presenting too much info. Everything you'd want to see to get a good idea of what I/O the system is performing on each request.

TRACE ASGI messages & other low level stuff
For when you really need to dig into something obscure, or understand exactly whats happening under the hood.

I can see this fitting in really well as a rule of thumb for other components, too. Eg. I'd like to see httpx issuing one-line-per-request at DEBUG level, or comprehensive logging of state changes at TRACE level. Looking at a web stack as a whole it feels like a natural split to me.

[tomchristie]

Somewhat related: encode/databases#156

[gvbgduh]

That's the great idea