[PROF-10422] Fix flaky GVL profiling integration spec

[ivoanjo]

What does this PR do?

This PR fixes a flaky specs seen in
https://app.circleci.com/pipelines/github/DataDog/dd-trace-rb/16487/workflows/df69cfde-1fd5-40dc-bff6-ac8a9d0bc713/jobs/593534 :

  1) Datadog::Profiling::Collectors::CpuAndWallTimeWorker#start when main thread is sleeping but a background thread is working when GVL profiling is enabled records Waiting for GVL samples
     Failure/Error: expect(waiting_for_gvl_time).to be_within(5).percent_of(total_time)
       expected 402832467 to be within 5% of 492191168

I was able to reproduce this failure on my local machine in around 1 in 20 runs.

This flakiness is caused by the profiler starting after a Waiting for GVL already started, and thus the profiler does not see it and still categorizes that period as "unknown"; thus our tight assertion is blown.

It's not a coincidence that the difference between 492191168 and 402832467 is within ~100ms: that's the scheduler latency caused by the background thread that's burning CPU.

To fix this issue, we subtract the duration of this initial period from the total_time: this is the period we care for.

To avoid us accidentally passing this test by having no Waiting for GVL, I've added an additional assertion for a sane total_value.

Motivation:

Make sure the profiler has zero flaky specs.

Additional Notes:

This PR is on top of #3934 for easier testing, but it's otherwise independent of it.

How to test the change?

Validate that CI is green and that the flaky test does not show up again.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.77%. Comparing base (c8d5351) to head (be8882a).

Additional details and impacted files

@@                             Coverage Diff                              @@
##           ivoanjo/fix-gvl-profling-exception-flaky    #3936      +/-   ##
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- Coverage                                     97.77%   97.77%   -0.01%     
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  Files                                          1297     1297              
  Lines                                         77973    77975       +2     
  Branches                                       3888     3888              
============================================================================
  Hits                                          76240    76240              
- Misses                                         1733     1735       +2     

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[pr-commenter]

Benchmarks

Benchmark execution time: 2024-09-20 12:23:44

Comparing candidate commit be8882a in PR branch ivoanjo/prof-10422-gvl-flaky-period with baseline commit c8d5351 in branch ivoanjo/fix-gvl-profling-exception-flaky.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 23 metrics, 2 unstable metrics.