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]

Benchmarks

Benchmark execution time: 2025-08-26 16:02:52

Comparing candidate commit 832d0a5 in PR branch dario.castane/er/issue-3639 with baseline commit c7f78ea in branch main.

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

[datadog-official]

⚠️ Tests

⚠️ Warnings

🧪 1 Test failed

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

Failed

=== RUN   TestSamplingDecision
    tracer_test.go:424: 
        	Error Trace:	/home/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
2025/08/26 15:53:19 Datadog Tracer v2.3.0-dev.1 ERROR: Failed to enable runtime metrics v2 err=runtimemetrics has already been started, 2 additional messages skipped (first occurrence: 26 Aug 25 15:53 UTC)
--- FAIL: TestSamplingDecision (0.10s)

ℹ️ Info

❄️ No new flaky tests detected

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