fix(profiling): fix data race when accessing span for thread

[nsrip-dd]

The ThreadSpanLinks singleton holds the active span (if one exists) for
a given thread ID. The get_active_span_from_thread_id member function
returns a pointer to the active span for a thread. The link_span member
function sets the active span for a thread. get_active_span_from_thread_id
accesses the map of spans under a mutex, but returns the pointer after
releasing the mutex, meaning link_span can modify the members of the
Span while the caller of get_active_span_from_thread_id is reading them.

Fix this by returning a copy of the Span. Use a std::optional to wrap
the return value of get_active_span_from_thread_id, rather than
returning a pointer. We want to tell whether or not there actually was a
span associated with the thread, but returning a pointer would require
us to heap allocate the copy of the Span.

TODO - regression test?
TODO - can we use C++17?

Checklist

• 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
• The PR description notes risks associated with the change, if any
• Newly-added code is easy to change
• The change follows the library release note guidelines
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

Reviewer Checklist

• Reviewer has checked that all the criteria below are met
• Title is accurate
• All changes are related to the pull request's stated goal
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Newly-added code is easy to change
• Release note makes sense to a user of the library
• If necessary, 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

[github-actions]

CODEOWNERS have been resolved as:

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

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-10-25 01:24:22

Comparing candidate commit dd868ee in PR branch nick.ripley/fix-stackv2-span-link-data-race with baseline commit 127e5b7 in branch main.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 387 metrics, 2 unstable metrics.

scenario:iast_aspects-rsplit_aspect

• 🟩 execution_time [-166.325ns; -139.613ns] or [-9.207%; -7.728%]