Add <log> directive to <system>

[repeatedly]

This directive controls fluentd's log behaviour
Now, support two parameters:

<system>
  <log>
    format json
    time_format %F
  </log>
</system>

• format: Change fluentd log format. text and json

text:

2017-07-27 07:37:02 +0900 [info]: #0 fluentd worker is now running worker=0

json:

{"time":"2017-07-27 06:44:54 +0900","level":"info","message":"fluentd worker is now running worker=0","worker_id":0}

• time_format: Change time format of log

time_format %Y

2017 [info]: #0 fluentd worker is now running worker=0

I have a plan to move some logging related parameters to under <log> like log_level.

[mururu]

Why not @format = :text?

[repeatedly]

Need to set other default values: https://github.com/fluent/fluentd/pull/1644/files#diff-d845866477e75d27b77c4b8acc63c7d9R163

[mururu]

I see.

[mururu]

I added a very small comment. Other than that LGTM.

[vkaverin]

Can't this log pattern be customizable as well?
For example, it may be necessary to have patten like [time][log_level] message to parse it easily.

It could be a customizable part of text format or a new format, let's say custom.

[repeatedly]

For example, it may be necessary to have patten like [time][log_level] message to parse it easily.

For easy to parse, format json is enough.

[vkaverin]

Will there be any documentation updates? Unfortunately, I'm not deeply familiar with the repository's structure, but I didn't where the docs are stored.

[repeatedly]

Visit fluentd-docs repository.