[APPSEC-57032] Add usr.login to Kit::AppSec::Events

[Strech]

What does this PR do?

1. Always set AppSec tag appsec.events.<event>.usr.login with a fallback value as usr.id
2. Pass usr.login when available to libddwaf

Motivation:

This is an intermediate step to comply with v3 ATO behavior in which it is expected (by the backend) that we alway pass login data appsec.events.<event>.usr.login and in addition we pass usr.login to libddwaf when available.

Change log entry

No. Announcement will be done as a part of #4433

Additional Notes:

Unfortunately, in the current state the Kit::AppSec::Events and Kit::Identity are coupled, which led to some questionable areas.

How to test the change?

CI should be fine

[datadog-datadog-prod-us1]

Datadog Report

Branch report: appsec-57032-add-user-login-to-identity
Commit report: d234fef
Test service: dd-trace-rb

✅ 0 Failed, 20643 Passed, 1370 Skipped, 3m 13.13s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-03-19 09:48:20

Comparing candidate commit d234fef in PR branch appsec-57032-add-user-login-to-identity with baseline commit 7d88b81 in branch master.

Found 3 performance improvements and 0 performance regressions! Performance is the same for 28 metrics, 2 unstable metrics.

scenario:line instrumentation - targeted

• 🟩 throughput [+8728.068op/s; +9199.385op/s] or [+5.608%; +5.911%]

scenario:line instrumentation - untargeted

• 🟩 throughput [+4779.085op/s; +5034.698op/s] or [+5.288%; +5.571%]

scenario:method instrumentation

• 🟩 throughput [+11383.841op/s; +11876.057op/s] or [+6.864%; +7.161%]

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.69%. Comparing base (7d88b81) to head (d234fef).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4508      +/-   ##
==========================================
- Coverage   97.70%   97.69%   -0.02%     
==========================================
  Files        1381     1381              
  Lines       83973    84017      +44     
  Branches     4251     4253       +2     
==========================================
+ Hits        82043    82077      +34     
- Misses       1930     1940      +10     

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