Access log vars

[nitrag]

Refers to #312

Add support for arbitrary request headers in access log via %(header{Header-Name})s atom

[nitrag]

@gi0baro ready for review, thanks

[yeongzhiwei]

hi @gi0baro , I would also love to have this feature to put it on parity with gunicorn. any progress on this MR?

[nalundgaard]

@nitrag does this address the BodyLength part of the request? It looks like that is not present in granian.

[nitrag]

@nalundgaard I believe there was additional overhead with adding body/content length, so I removed it. I'm hoping this PR is low enough impact so it has higher chance of being merged.

@gi0baro

[gi0baro]

Hey all, please stop pinging me.
If I leave a PR open and don't reject it, it doesn't mean I won't merge it.

A lot happened in my personal life in the past few months – and even if this wasn't the case – give me the time to appropriately review stuff.
When I'll be able to review this, I'll do.

[gi0baro]

I refactored this, specifically:

• dropped response headers entirely, too much complexity added for a limited use-case
• use %(header{Header-Name})s as the atom, every other atom is not compatible with Gunicorn, thus no reason to use the same fmt just for this one
• removed user-agent as a special case, it's an header like everything else
• split all the code paths for header vs non-header cases
• dropped tests, I don't see the value gain on an already slow suite
• updated README accordingly