Structured JSON logging

[igorwwwwwwwwwwwwwwwwwwww]

Description

GitLab SRE here. :)

This patch introduces support for logging in JSON format (via new --log-json flag), and also changes the request logging to be properly structured.

This allows for easier diagnosis when there is an issue in production, as we can parse the fields out into Elasticsearch (or other log processing tools) and filter on them.

Before:

{"time": "2021-05-11T17:26:16.493664000+02:00", "level": "D", "msg": "client request", "extra": {"req": "&{GET /a/b HTTP/1.1 1 1 map[Accept:[*/*] User-Agent:[curl/7.64.1] Via:[go-camo]] {} <nil> 0 [] false go-camo map[] map[] <nil> map[] 127.0.0.1:52466 /a/b <nil> <nil> <nil> 0xc000030e80}"}}

After:

{"time": "2021-05-11T17:33:44.199222000+02:00", "level": "D", "msg": "client request", "extra": {"proto": "HTTP/1.1", "header": "map[Accept:[*/*] User-Agent:[curl/7.64.1] Via:[go-camo]]", "transfer_encoding": "[]","method": "GET", "path": "/a/b", "content_length": "0", "host": "go-camo", "remote_addr": "127.0.0.1:52613"}}

There's still room for improvement wrt nested maps, but this is already a lot more machine friendly.

Checklist

• [x] Code compiles correctly
• [ ] Created tests (if appropriate)
• [x] All tests passing
• [x] Extended the README / documentation (if necessary)

[dropwhile]

lgtm. Thanks!