Fix curl integration race condition

[iamluc]

Description

Spotted by the CI, though not frequent.
e.g. https://app.circleci.com/pipelines/github/DataDog/dd-trace-php/17462/workflows/f2a5d19c-4f05-4f57-aed9-f400f6677938/jobs/5150058

Fix a race condition where curl_multi_exec is called many times while finished ($still_running at false) before curl_multi_info_read.
In that case, the spans could have been removed from the storage.
+
Remove weird code using the last instance of a previous loop value ($requestSpan)

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.09%. Comparing base (ebaa1ca) to head (1133a8a).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2847      +/-   ##
============================================
+ Coverage     81.06%   81.09%   +0.02%     
+ Complexity     2516     2515       -1     
============================================
  Files           146      146              
  Lines         14653    14648       -5     
============================================
  Hits          11879    11879              
+ Misses         2774     2769       -5     

Flag	Coverage Δ
tracer-extension	78.18% <ø> (ø)
tracer-php	82.31% <100.00%> (+0.03%)	⬆️

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

Files with missing lines	Coverage Δ
src/DDTrace/Integrations/Curl/CurlIntegration.php	95.23% <100.00%> (+2.75%)	⬆️

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ebaa1ca...1133a8a. Read the comment docs.

[bwoebi]

The difference is essentially that $saveSpans gets always set. I think that's right.

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-09-12 12:24:53

Comparing candidate commit c4c916f in PR branch luc/fix-curl-integration-race-condition with baseline commit ebaa1ca in branch master.

Found 1 performance improvements and 5 performance regressions! Performance is the same for 172 metrics, 0 unstable metrics.

scenario:BM_TeaSapiSpindown

• 🟥 execution_time [+13.381µs; +28.847µs] or [+2.585%; +5.573%]

scenario:EmptyFileBench/benchEmptyFileBaseline-opcache

• 🟥 execution_time [+71.938µs; +208.202µs] or [+2.494%; +7.218%]

scenario:LaravelBench/benchLaravelBaseline-opcache

• 🟥 execution_time [+62.072µs; +209.768µs] or [+2.063%; +6.972%]

scenario:SamplingRuleMatchingBench/benchRegexMatching2

• 🟥 execution_time [+152.622ns; +548.178ns] or [+2.135%; +7.668%]

scenario:SamplingRuleMatchingBench/benchRegexMatching4

• 🟥 execution_time [+266.970ns; +701.630ns] or [+3.737%; +9.820%]

scenario:WordPressBench/benchWordPressOverhead

• 🟩 execution_time [-21.911ms; -21.621ms] or [-83.229%; -82.125%]