add recipe for handling flaky tests

[emmettbutler]

What does this PR do?

This change adds brief and generic documentation about how to handle test unreliability

Motivation:

This change aims to make it easier for new contributors to become effective in this repo.

Change log entry

Additional Notes:

How to test the change?

[github-actions]

👋 Hey @DataDog/ruby-guild, please fill "Change log entry" section in the pull request description.

If changes need to be present in CHANGELOG.md you can state it this way

**Change log entry**

Yes. A brief summary to be placed into the CHANGELOG.md

(possible answers Yes/Yep/Yeah)

Or you can opt out like that

**Change log entry**

None.

(possible answers No/Nope/None)

^{Visited at: 2025-06-24 12:11:48 UTC}

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.55%. Comparing base (4262c44) to head (565ee3d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4752   +/-   ##
=======================================
  Coverage   97.55%   97.55%           
=======================================
  Files        1486     1486           
  Lines       88592    88592           
  Branches     4589     4589           
=======================================
+ Hits        86423    86425    +2     
+ Misses       2169     2167    -2     

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[datadog-datadog-prod-us1]

Datadog Report

Branch report: emmett.butler/flaky-guidance
Commit report: 565ee3d
Test service: dd-trace-rb

✅ 0 Failed, 21774 Passed, 1296 Skipped, 5m 23.44s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-06-25 15:46:04

Comparing candidate commit 565ee3d in PR branch emmett.butler/flaky-guidance with baseline commit 4262c44 in branch master.

Found 0 performance improvements and 2 performance regressions! Performance is the same for 43 metrics, 2 unstable metrics.

scenario:error - without error tracking with http

• 🟥 throughput [-28.561op/s; -21.420op/s] or [-19.799%; -14.849%]

scenario:profiling - profiler gc

• 🟥 throughput [-19882.866op/s; -19367.394op/s] or [-5.398%; -5.258%]

[p-datadog]

We don't presently seem to run any tests for contributor PRs automatically (e.g. see: #4543). An employee has to push the code to this repo (which contribuotrs have no permission to do) for CI to run. I don't know if contributors will be able to restart any failing jobs either.

[TonyCTHsu]

@p-datadog , @emmettbutler is employed at Datadog. The Gitlab job failed without useful information only doc changes pull request.