**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
```ruby
# 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.

---------

Signed-off-by: Shizuo Fujita <[email protected]>
Co-authored-by: Daijiro Fukuda <[email protected]>