fix(tracing): avoid assigning span's local root to self, so that the python GC doesn't kick in [backport 2.12]

[github-actions]

Backport e160b36 from #10809 to 2.12.

<<Description copied from the original PR>>

We (Modal Labs) noticed that enabling tracing in parts of our code caused the Python GC to kick in a lot and add large delays to our event loop. A simple repro (see test) showed us that there was a self reference cycle in span when a top-level span's local root was assigned to the span itself, preventing Python from cleaning up the objects by reference counting.

The change itself is pretty straightforward - we use None to represent when a span's local root is itself. The span still exposes _local_root with the same behavior via a property, and internally stores a _local_root_value that holds a non-self-referential span. This also means that the tracer no longer needs to explicitly set the local root for top-level spans. This should be fairly non-risky as it doesn't change any code behavior.

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

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: backport-10809-to-2.12
Commit report: d89e748
Test service: dd-trace-py

✅ 0 Failed, 105150 Passed, 59423 Skipped, 3h 2m 30.2s Total duration (5h 29m 37.15s time saved)

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-09-27 13:53:10

Comparing candidate commit d89e748 in PR branch backport-10809-to-2.12 with baseline commit db5721c in branch 2.12.

Found 6 performance improvements and 0 performance regressions! Performance is the same for 347 metrics, 47 unstable metrics.

scenario:otelspan-add-metrics

• 🟩 max_rss_usage [-9.778MB; -9.544MB] or [-27.491%; -26.834%]

scenario:otelspan-add-tags

• 🟩 max_rss_usage [-9.844MB; -9.612MB] or [-27.620%; -26.969%]

scenario:span-add-metrics

• 🟩 max_rss_usage [-17.982MB; -17.782MB] or [-36.180%; -35.776%]

scenario:span-add-tags

• 🟩 max_rss_usage [-4.728MB; -4.539MB] or [-16.265%; -15.612%]

scenario:span-start

• 🟩 execution_time [-2.186ms; -1.740ms] or [-11.084%; -8.819%]

scenario:span-start-traceid128

• 🟩 execution_time [-2.219ms; -1.775ms] or [-10.920%; -8.736%]

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 33.33333% with 14 lines in your changes missing coverage. Please review.

Please upload report for BASE (2.12@9e2f85d). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
ddtrace/_trace/span.py	42.85%	8 Missing ⚠️
tests/tracer/test_tracer.py	0.00%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             2.12   #10836   +/-   ##
=======================================
  Coverage        ?    9.84%           
=======================================
  Files           ?     1411           
  Lines           ?   121005           
  Branches        ?        0           
=======================================
  Hits            ?    11918           
  Misses          ?   109087           
  Partials        ?        0           

Flag	Coverage Δ
	9.84% <33.33%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.