Add ext_monitor gem to improve Monitor class's operations

[ganmacs]

Which issue(s) this PR fixes:
Fixes #2645

What this PR does / why we need it:

From ruby 2.6.5, Monitor class became slower due to fixing a deadlock bug (https://bugs.ruby-lang.org/issues/15992).
https://github.com/nurse/ext_monitor/ mitigates performance for ruby 2.6.5 or later and improves it for prior to 2.6.4 by 2x(#2645 (comment)).

Docs Changes:

no need

Release Note:

same as the title

[ganmacs]

I'll merge this after #2380

[repeatedly]

By introducing ext_monitor in ruby 2.7, how about changing to if ext_monitor is installed, fluentd uses it like oj? It reduces the dependencies and fluentd works even if ext_monitor has a problem.

[ganmacs]

Sorry, I had a mistake about a condition of installing.
Ruby 2.7 is bundled ext_monitor like mitigation code by default so 2.7 is fast enough without ext_monitor.

It reduces the dependencies and fluentd works even if ext_monitor has a problem.

Does it mean like below? and removing gem.add_runtime_dependency("ext_monitor", [">= 0.1.1", "< 0.2"]) from gemspec?

if Gem::Version.create(RUBY_VERSION) >= Gem::Version.create('2.7.0')
  require 'monitor'
else
  begin
    require 'ext_monitor'
  rescue LoadError => _
    require 'monitor'
  end
end

[repeatedly]

Does it mean like below?

Yes.

[ganmacs]

fixed.
4ae5416

This log always outputs to STDOUT without ext_monitor. it might be noisy. what do you think of that? @repeatedly

[repeatedly]

Maybe, such message is not needed.
td-agent and docker images will install ext_monitor.
So most users don't have Monitor issue.

Add to instruction to gem install article is enough.

[ganmacs]

Maybe, such message is not needed.

👍

fixed 0a772f2