Access log vars

[nitrag]

Resolves #312

Adds the ability to implement UserAgent and custom request/response headers to the access log.

• Add user_agent access log variable for logging the request User-Agent header
• Add support for arbitrary request headers via %({Header-Name}i)s syntax (gunicorn-compatible)
• Add support for arbitrary response headers via %({Header-Name}o)s syntax (gunicorn-compatible)
• Validate access log format strings eagerly at startup to catch misconfiguration early
• Response header capture overhead is gated behind a needs_resp_headers flag — zero cost when no %({...}o)s atoms are in the format string

[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.