Add support for JSON formatted logs

[chagui]

Description

This pull-request introduce a configuration option to set the log format to JSON. The option is available both via the config file and CLI arguments.

Solves #3133

How to use it

config file entry

[log]
  format = "json"

CLIs argument:

• buildkitd --log-format json
• buildctl --log-format json

Notes

Original work from @moeghanem and @benjamin010095. I forked #3824 and squashed the commits into a single one.
See also:

• Buildkitd and Buildctl json log support #3824
• Buildctl --opt flag documentation, Buildkitd and Buildctl json log support #3823
• Buildctl --opt flag documentation, Buildkitd and Buildctl json log support #3822

[jedevc]

Thanks for carrying this @chagui 🎉 Left a couple comments 👍

[moeghanem]

Thanks for carrying this @chagui 🎉 Left a couple comments 👍

Yes, thanks a lot @chagui. This completely slipped my mind 😅

[crazy-max]

For buildkitd I wonder if we should have a LogConfig struct with Format attr instead of just LogFormat so we could extend log config in the future such as caller bool that would add file:line of the caller to log output. I was also thinking of a level one as well that would be overloaded by Debug if set.

In TOML config it would look like:

[log]
  level = "info"
  format = "text"
  caller = false

[chagui]

I really like the idea, I will update the code accordingly.

[crazy-max]

Please avoid nested struct and create a dedicated type LogConfig like HistoryConfig.

[chagui]

Done.
Out of curiosity what is the rational for avoiding nested struct?
I saw the Workers nested struct and thought to do the same because Log would be a simple one.

[jedevc]

PTAL @crazy-max