Implement standard compatible severity formatting to LevelFluentLogger

[saidie]

Hi,

The recently added standard logger compatible LevelFluentLogger is great!
I've been tried to use it in my project and noticed that its use of formatter is incompatible with standard one.

That is, format_severity, which is a private method of ::Logger, is used inside the formatter here. This could be simply because format_severity is not applied here.
In the standard ::Logger implementation, format_severity is applied before passing a severity to formatter like this.

This PR implements standard log formatter compatible way of severity formatting.

[sonots]

LGTM. @toyama0919 ping?

[saidie]

Thank you!
Now, I'm trying to fix the test 🤔

[saidie]

It seems to fail due to recent master branch updates of https://github.com/fluent/fluentd

[toyama0919]

The recently added standard logger compatible LevelFluentLogger is great!

😊

LGTM.

[saidie]

Thank you for LGTM 😊

[saidie]

By manual binary search, I found that the test failure is caused by this fluentd PR.
And all test cases are passed in my machine when WAIT environment variable has longer value, e.g., 0.5. Maybe we need to make ENV['WAIT'] default value longer as with the similar commit in the PR ?

[repeatedly]

That's weird. It means the response time of new in_forward is longer than old in_forward?

[saidie]

I'm not well understood the detail of the fluentd PR but it sounds weird too that new plugin is slower.
ENV['WAIT'] is used also for waiting dummy fluentd startup here. This could be a core of the problem ??

[repeatedly]

This could be a core of the problem ??

If increasing wait time resolves test failures,
it is one temporarily approach.

[saidie]

By my investigation, the problem is that the posted content has not yet been read by in_forward plugin when DummyFluentd#output is called. This is occurred because Cool.io event loop takes some time here (about 0.5s in my machine).
It seems to be inevitable to increase wait time at this point. I will create another PR for that tomorrow (or could I include it in this PR?).

[repeatedly]

Sorry for the late. Just merged.