refactor(tracer/trace): add lock assertions to trace struct

[kakkoyun]

What does this PR do?

Motivation

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running ./scripts/lint.sh locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

Unsure? Have a question? Request a review!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.06%. Comparing base (86c9a30) to head (5257eb3).

Additional details and impacted files

Files with missing lines	Coverage Δ
ddtrace/tracer/propagating_tags.go	68.08% <100.00%> (+0.69%)	⬆️
ddtrace/tracer/spancontext.go	88.17% <100.00%> (+0.14%)	⬆️

... and 4 files with indirect coverage changes

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[pr-commenter]

Benchmarks

Benchmark execution time: 2026-01-23 12:26:17

Comparing candidate commit 5257eb3 in PR branch kakkoyun/LANGPLAT-434/assert_trace with baseline commit 86c9a30 in branch main.

Found 0 performance improvements and 13 performance regressions! Performance is the same for 142 metrics, 9 unstable metrics.

scenario:BenchmarkExtractW3C-25

• 🟥 execution_time [+102.366ns; +147.634ns] or [+4.398%; +6.344%]

scenario:BenchmarkInjectW3C-25

• 🟥 execution_time [+131.136ns; +148.264ns] or [+5.806%; +6.564%]

scenario:BenchmarkOTelApiWithCustomTags/datadog_otel_api-25

• 🟥 execution_time [+191.842ns; +221.158ns] or [+4.540%; +5.234%]

scenario:BenchmarkOTelApiWithCustomTags/otel_api-25

• 🟥 execution_time [+216.974ns; +242.426ns] or [+3.688%; +4.121%]

scenario:BenchmarkPartialFlushing/Disabled-25

• 🟥 execution_time [+27.137ms; +29.317ms] or [+10.256%; +11.080%]

scenario:BenchmarkPartialFlushing/Enabled-25

• 🟥 execution_time [+29.342ms; +30.707ms] or [+10.947%; +11.457%]

scenario:BenchmarkSingleSpanRetention/no-rules-25

• 🟥 execution_time [+31.314µs; +31.915µs] or [+13.185%; +13.438%]

scenario:BenchmarkSingleSpanRetention/with-rules/match-all-25

• 🟥 execution_time [+30.500µs; +31.963µs] or [+12.573%; +13.175%]

scenario:BenchmarkSingleSpanRetention/with-rules/match-half-25

• 🟥 execution_time [+31.091µs; +32.382µs] or [+12.858%; +13.391%]

scenario:BenchmarkStartSpan-25

• 🟥 execution_time [+211.254ns; +228.746ns] or [+12.775%; +13.832%]

scenario:BenchmarkStartSpanConfig/scenario_WithStartSpanConfig-25

• 🟥 execution_time [+198.497ns; +208.103ns] or [+6.984%; +7.322%]

scenario:BenchmarkStartSpanConfig/scenario_none-25

• 🟥 execution_time [+200.348ns; +220.252ns] or [+6.645%; +7.305%]

scenario:BenchmarkTracerAddSpans-25

• 🟥 execution_time [+298.894ns; +325.306ns] or [+7.317%; +7.963%]

[darccio]

The impact on performance looks significant. Could it be flakiness?

[kakkoyun]

The impact on performance looks significant. Could it be flakiness?

Quite possible. Until I know for sure. I'll mark it as draft.