Add support for JSON logs

[icco]

I would love if there were a JSON log formatting option. Logs are currently plain strings which can be hard to dynamically parse with some services.

[manugarg]

#462 may fix this. Not committed yet.

[manugarg]

@icco This is done in the code in the repo. You can use the flag --logfmt=json flag to export logs in JSON format.

Ways to use this unreleased version: "main" docker tag, or the "tip" release. It will go out in the v0.12.9 release, later this month.

[icco]

Thanks we're rolling it out today!