fix(ddtrace): ensure sampling is propagated on OpenTelemetry

[darccio]

What does this PR do?

Fixes #3639.

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!

[pr-commenter[bot]]

Benchmarks

Benchmark execution time: 2025-12-02 09:32:26

Comparing candidate commit 4a29ea64294cdf4ae38841ce830e98acc0c42271 in PR branch dario.castane/er/issue-3639 with baseline commit f7d92e7439000bdc929d0a5b0d691290792b3b0a in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 24 metrics, 0 unstable metrics.

[datadog-official[bot]]

⚠️ Tests

⚠️ Warnings

❄️ 1 New flaky test detected

TestGlobalFunctions from github.com/DataDog/dd-trace-go/v2/internal/orchestrion/_integration/net_http (Datadog)

Failed

=== RUN   TestGlobalFunctions
    generated_test.go:25: Running setup
    agent.go:78: mockagent: starting
    base.go:35: 
        	Error Trace:	/home/runner/work/dd-trace-go/dd-trace-go/dd-trace-go/internal/orchestrion/_integration/net_http/base.go:35
        	            				/opt/hostedtoolcache/go/1.24.10/x64/src/runtime/asm_amd64.s:1700
        	Error:      	Target error should be in err chain:
        	            	expected: "http: Server closed"
...

🧪 1 Test failed

TestSamplingDecision from github.com/DataDog/dd-trace-go/v2/ddtrace/opentelemetry (Datadog)

Failed

=== RUN   TestSamplingDecision
    tracer_test.go:424: 
        	Error Trace:	/Users/runner/work/dd-trace-go/dd-trace-go/ddtrace/opentelemetry/tracer_test.go:424
        	Error:      	Should be true
        	Test:       	TestSamplingDecision
        	Messages:   	parent span is sampled, but child span is not sampled
2025/12/02 09:15:55 Datadog Tracer v2.5.0-dev ERROR: lost 1 traces: Post "http://localhost:8126/v0.4/traces": dial tcp 127.0.0.1:8126: connect: connection refused, 4 additional messages skipped (first occurrence: 02 Dec 25 09:15 UTC)
--- FAIL: TestSamplingDecision (0.04s)

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[mxderouet]

Hi team do we plan to deploy this? Is there a card where I can follow the progress? Thanks!

[darccio]

@mxderouet This PR wasn't intended to be merged. It was part of an investigation by APM DCS LP. SDK took over and I think the best solution, already in review, is #4238.

I'm closing this one as it's no longer relevant.