Handle log timestamps in Ansi2Json

What does this MR do and why?

This MR improves Gitlab::Ci::Ansi2json::Converter so that it parses and outputs log timestamps generated by gitlab-runner when the FF_TIMESTAMPS feature flag is enabled.

This MR is probably easier to review commit-by-commit.

The processing of timestamps in this MR introduces a performance penalty of ~8% when compared with master, but given that this is behind a gitlab-runner FF, it seems OK. I've created a follow-up issue to address the performance issues, before we roll this out more widely.

Logs are processed by the Gitlab::Ci::Ansi2html and Gitlab::Ci::Ansi2json classes, to produce HTML and JSON variants. Recently, the runner has introduced a FF_TIMESTAMPS feature flag, which will cause it to emit lines with a slightly modified logic:

E.g. 2024-05-14T11:19:20.000000Z 00O+Hey there!. The first 32 characters are a timestamp prefix, and must be treated as optional. If present, Ansi2json must include each line's respective timestamp in its output.

Format:

<timestamp> <stream number><stream type><full line type>

• timestamp: UTC RFC3339 timestamp
• stream number: 1 byte (2 hex chars) stream number
• stream type: E/O (Err or Out)
• full line type: + if line is continuation of previous line, otherwise

Closes: #455581 (closed)

MR acceptance checklist

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Screenshots or screen recordings

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Job without FF_TIMESTAMPS (no change)

Before	After

Job with FF_TIMESTAMPS (timestamps removed)

Before	After

Performance benchmarks

runs = 100; Benchmark.bm do |x|
  x.report('Large log with timestamps (this branch)') do
    runs.times do 
      json = File.open("~/Downloads/raw-large.txt", "r") { |stream| converter = Gitlab::Ci::Ansi2json::Converter.new.convert(stream, nil) }
    end
  end
  x.report('Large log without timestamps (this branch)') do
    runs.times do
      json = File.open("~/Downloads/raw-no-ts-large.txt", "r") { |stream| converter = Gitlab::Ci::Ansi2json::Converter.new.convert(stream, nil) }
    end
  end
end

Scenario	user	system	total	real
Large log without timestamps (master)	50.721283	0.442331	51.163614	51.914806
Large log with timestamps (master)	52.246681	0.502974	52.749655	53.976508
Large log without timestamps (this branch)	52.938921	0.364169	53.303090	53.849536
Large log with timestamps (this branch)	57.927132	0.381748	58.308880	58.697001

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.