chore(ci_visibility): add threading and multiprocessing coverage tests

[romainkomorndatadog]

Adds some tests for the internal coverage tool.

Also fixes a bug where coverage absorbed from threading or multiprocessing modules was being added to the session-level coverage even when said coverage wasn't enabled.

No release note because this is experimental and unreleased.

Checklist

• Change(s) are motivated and described in the PR description
• Testing strategy is described if automated tests are not included in the PR
• Risks are described (performance impact, potential for breakage, maintainability)
• Change is maintainable (easy to change, telemetry, documentation)
• Library release note guidelines are followed or label changelog/no-changelog is set
• Documentation is included (in-code, generated user docs, public corp docs)
• Backport labels are set (if applicable)
• If this PR changes the public interface, I've notified @DataDog/apm-tees.

Reviewer Checklist

• Title is accurate
• All changes are related to the pull request's stated goal
• Description motivates each change
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Change is maintainable (easy to change, telemetry, documentation)
• Release note makes sense to a user of the library
• Author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-06-11 10:11:32

Comparing candidate commit 638baa8 in PR branch romain.komorn/CIVIS-9428/add_coverage_tests with baseline commit 360b469 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 221 metrics, 9 unstable metrics.

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: romain.komorn/CIVIS-9428/add_coverage_tests
Commit report: ecd6a58
Test service: dd-trace-py

✅ 0 Failed, 119008 Passed, 59251 Skipped, 3h 57m 54.03s Total duration (3h 32m 0.93s time saved)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 20.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 10.27%. Comparing base (cd92beb) to head (ecd6a58).
Report is 2 commits behind head on main.

Files	Patch %	Lines
ddtrace/internal/coverage/code.py	12.50%	7 Missing ⚠️
ddtrace/internal/coverage/instrumentation.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #9479       +/-   ##
===========================================
- Coverage   75.89%   10.27%   -65.62%     
===========================================
  Files        1315     1285       -30     
  Lines      124966   123124     -1842     
===========================================
- Hits        94837    12651    -82186     
- Misses      30129   110473    +80344     

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