2.0.0-beta.5 removes `log.format` flag

[TimSimmons]

What did you do?

Upgraded from beta.3 to beta.5

What did you expect to see?

The --log.format flag continuing to exist.

What did you see instead? Under which circumstances?

prometheus: error: unknown long flag '--log.format'

Environment

• Prometheus version:

$ ./bin/prometheus --version
prometheus, version 2.0.0-beta.5 (branch: HEAD, revision: 64c7932e851f3276391fc3e216cd30f447d4924e)
  build user:       root@df40cc0bd79b
  build date:       20170921-16:23:04
  go version:       go1.8.3

Additional Context

#promtheus
11:10 T<timsim> Were the changes from beta.4 to beta.5 supposed to remove the `--log.format` flag? https://github.com/prometheus/prometheus/pull/3152 It appears they have
11:11 G<gouthamve[m]> Yes. We were initially going to see if people really need another format and slowly add it back I guess
11:11 G<gouthamve[m]> Can you create an issue?

I was using --log.format="logger:syslog?appname=prometheus&local=7&json=true". If there's another way to do that in 2.0 that'll be preferred I'm happy to change. I suppose it could also be done with process supervisors or something. However, I would expect people upgrading from 1.x will be used to this flag/format and will have something that works for them.

[khardsonhurley]

I am having the same issue with --log.format on version 2.0.0 and was wondering if this issue has been addressed?

[krasi-georgiev]

this was removed when the Prometheus started using the go kit logger.
I had a quick look and it seems that go kit also support the json format.

@khardsonhurley what is your use case why do you need a different log format?

[khardsonhurley]

Nevermind, we just removed it and the startout format works just fine for our use case. Thanks!

[krasi-georgiev]

ok , thanks , closing for now , but feel free to reopen if there is a good use case.

[TimSimmons]

Sorry, maybe I'm missing something obvious. Is there a way to log with JSON now? I don't see a flag or a configuration option for changing the logging format.

[krasi-georgiev]

no there isn't , go kit seems to support json formatting so in theory shouldn't be a big problem to add the flag given there are enough use cases that need this.

@TimSimmons what is your use case?

[TimSimmons]

I'd like to have structured logging in JSON so that the messages can be optimally indexed in Elasticsearch in our centralized logging setup.

[krasi-georgiev]

yep that sound like a good use case :)
I hope you have some time to open a PR 👍

looking at the example looks like an easy task, just need to use NewJSONLogger
https://github.com/go-kit/kit/tree/master/log

[alexander-yu]

If nobody's working on this, I'd like to take this up!

[krasi-georgiev]

@alexander-yu yep go ahead. Thanks. I will review when ready.

[ntindall]

Ping on this - would love to see this happen @alexander-yu

[alexander-yu]

Sure thing; promlog needs to be modified first to get support for using JSON loggers. I've got a PR waiting on prometheus/common (see above) that'll provide the necessary methods for configuring the logger type.

[ntindall]

Nice 👍