fix(profiling): join v2 sampling thread on exit

[taegyunkim]

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• PR author has checked that all the criteria below are met
• The PR description includes an overview of the change
• The PR description articulates the motivation for the change
• The change includes tests OR the PR description describes a testing strategy
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• Newly-added code is easy to change
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• Reviewer has checked that all the criteria below are met
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[github-actions]

CODEOWNERS have been resolved as:

ddtrace/internal/datadog/profiling/stack_v2/include/sampler.hpp         @DataDog/profiling-python
ddtrace/internal/datadog/profiling/stack_v2/src/sampler.cpp             @DataDog/profiling-python

[github-actions]

Bootstrap import analysis

Comparison of import times between this PR and base.

Summary

The average import time from this PR is: 236 ± 2 ms.

The average import time from base is: 237 ± 2 ms.

The import time difference between this PR and base is: -1.42 ± 0.08 ms.

Import time breakdown

The following import paths have shrunk:

ddtrace.auto 1.926 ms (0.82%)

ddtrace.bootstrap.sitecustomize 1.254 ms (0.53%)

ddtrace.bootstrap.preload 1.254 ms (0.53%)

ddtrace.internal.remoteconfig.client 0.629 ms (0.27%)

ddtrace 0.672 ms (0.28%)

ddtrace.internal._unpatched 0.022 ms (0.01%)

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-05-30 22:03:32

Comparing candidate commit d409710 in PR branch taegyunkim/v2-join with baseline commit 7425abd in branch main.

Found 0 performance improvements and 2 performance regressions! Performance is the same for 501 metrics, 5 unstable metrics.

scenario:iastaspectsospath-ospathnormcase_aspect

• 🟥 execution_time [+393.990ns; +449.270ns] or [+11.413%; +13.015%]

scenario:telemetryaddmetric-1-distribution-metric-1-times

• 🟥 execution_time [+267.473ns; +330.014ns] or [+9.324%; +11.504%]

[nsrip-dd]

IIRC @P403n1x87 had some concern about whether this would slow down application exit too much? So we probably want his review on this.

[taegyunkim]

@nsrip-dd Yeah, I'm also debating whether this is really necessary or not. I initially had thought whether this would fix some of the read-after-free problems we see from segfaults, but don't really have clear evidences.

[realFlowControl]

IIRC @P403n1x87 had some concern about whether this would slow down application exit too much? So we probably want his review on this.

Just my two cents: IMHO it is the right thing to join this thread.
From a quick scan over the code, it looks like we're delaying the end of the application by a maximum of a two-digit number of milliseconds? That sounds reasonable to me.