benchmark: Use IO#size instead of File.size to speed up file generation on Windows

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
Using File.size(path) in a tight loop is very slow, especially on Windows,
because it resolves the path and calls stat every time.

• benchmark code

require 'bundler/inline'
gemfile do
  source 'https://rubygems.org'
  gem 'benchmark-ips'
end

FILE_PATH = "dummy_file.txt"

File.open(FILE_PATH, "w") do |f|
  Benchmark.ips do |x|
    x.time = 10

    x.report("File.size(path)") { File.size(FILE_PATH) }
    x.report("File#size") { f.size }
  end
end

• result

ruby 4.0.1 (2026-01-13 revision e04267a14b) +PRISM [x64-mingw-ucrt]
Warming up --------------------------------------
     File.size(path)     3.104k i/100ms
           File#size    17.874k i/100ms
Calculating -------------------------------------
     File.size(path)     31.099k (± 1.5%) i/s   (32.16 μs/i) -    313.504k in  10.083209s
           File#size    182.821k (± 1.4%) i/s    (5.47 μs/i) -      1.841M in  10.072168s

Using the instance method IO#size (f.size) avoids this overhead and
speeds up the 1GB test file generation by about 5x (from ~45s to ~9s) in

fluentd/tasks/benchmark.rb

Lines 13 to 23 in b819ccf

	task :prepare_1GB do
	FileUtils.mkdir_p(File.dirname(BENCHMARK_FILE_PATH))
	File.open(BENCHMARK_FILE_PATH, "w") do |f|
	data = { "message": "a" * 1024 }.to_json
	loop do
	f.puts data
	break if File.size(BENCHMARK_FILE_PATH) > BENCHMARK_FILE_SIZE
	end
	end
	end

Docs Changes:
N/A

Release Note:
N/A