metrics: enable input metrics by default

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
I ran fluend with --enable-input-metrics option and read 10GB file with the in_tail plugin
There was no significant performance degradation.

So, I think it is safe to always enable --enable-input-metrics.

environment

• ruby 3.4.3 (2025-04-14 revision d0b7e5b6a0) +PRISM [x86_64-linux]

results

• fluentd master branch (efbb51d)
  • ruby bin/fluentd -c in_tail.conf
    • 65.939232375 seconds
  • ruby bin/fluentd --enable-input-metrics -c in_tail.conf
    • 67.776236261 seconds
• fluentd 1.16.9 (daccbc6)
  • ruby bin/fluentd -c in_tail.conf
    • 106.312500419 seconds

config

<source>
  @type tail
  tag test
  path "#{File.expand_path '~/tmp/fluentd/access-*.log'}"
  read_from_head true
  <parse>
    @type json
  </parse>
</source>

<match **>
  @type file
  path "#{File.expand_path '~/tmp/fluentd/log'}"
</match>

script to generate 10GB

# frozen_string_literal: true
require "json"
require "fileutils"

def data_generater(str)
  {
    "message": str * 1000,
  }.to_json
end

FILE_MAX_SIZE = 10 * 1024 * 1024 * 1024
FILE_PATH = "/home/watson/tmp/fluentd/access-1.log"

dir = File.dirname(FILE_PATH)
FileUtils.mkdir_p(dir)

File.open(FILE_PATH, "w") do |f|
  data = data_generater('a')
  loop do
    f.puts data
    break if File.size(FILE_PATH) > FILE_MAX_SIZE
  end
end

Docs Changes:
fluent/fluentd-docs-gitbook#578

Release Note:
The same as the title.

[cosmo0920]

I'm pleased this feature to be enabled by default. Thank you for your work! 👍

[daipom]

It would be better to add some tests to check if the option for disabling the feature works intendedly.

[daipom]

Thanks!
Could you update this test as well?

fluentd/test/test_supervisor.rb

Line 57 in a3f4c82

def test_system_config

[daipom]

LGTM. Thanks!