[APPSEC-56960] Add collection of request headers for identity

[Strech]

What does this PR do?
Two things:

1. Add new event for Rack contrib named rack.request.finish
2. Add subscriber to the said event to set tags of identity related headers (see User event)

Motivation:

This will be a part of the #4433 and could be extracted without a fear of breaking anything.

Change log entry

No. It will be announced as of #4433 release

Additional Notes:

Unfortunately, we don't have time to go over the current published/subscriber model and it has very odd limitations. Hence the code is placed in the most convenient way, but not in the most right.

How to test the change?

CI is good enough.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.69%. Comparing base (f7b904d) to head (f5cff78).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4468      +/-   ##
==========================================
- Coverage   97.70%   97.69%   -0.01%     
==========================================
  Files        1376     1378       +2     
  Lines       83824    83898      +74     
  Branches     4251     4253       +2     
==========================================
+ Hits        81897    81967      +70     
- Misses       1927     1931       +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[datadog-datadog-prod-us1]

Datadog Report

Branch report: appsec-56960-add-collection-of-rack-request-headers-to-identity
Commit report: f5cff78
Test service: dd-trace-rb

✅ 0 Failed, 20432 Passed, 1369 Skipped, 3m 15.16s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-03-08 11:25:38

Comparing candidate commit f5cff78 in PR branch appsec-56960-add-collection-of-rack-request-headers-to-identity with baseline commit f7b904d in branch master.

Found 2 performance improvements and 0 performance regressions! Performance is the same for 29 metrics, 2 unstable metrics.

scenario:tracing - Propagation - Datadog

• 🟩 throughput [+3361.952op/s; +3434.687op/s] or [+11.475%; +11.724%]

scenario:tracing - Tracing.log_correlation

• 🟩 throughput [+8194.009op/s; +8517.301op/s] or [+7.498%; +7.794%]