fix(ddtrace/opentelemetry): propagate sampling decision to child spans

[darccio]

What does this PR do?

Propagates the parent context's sampling decision to child spans if parent has been sampled.

Motivation

Closes #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 golangci-lint run 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.

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[pr-commenter]

Benchmarks

Benchmark execution time: 2025-06-20 07:54:21

Comparing candidate commit 2e84c0b in PR branch dario.castane/langplat-603/sampling-decision-fix with baseline commit 2bcfa37 in branch main.

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

[darccio]

Converting to draft as @mtoffl01 pointed off-line some good reasons about this PR duplicating logic deep in the chain.

[sdwa0]

This doesn't look right.

If the parent span is sampled, the new span shouldn't need to get force-sampled to get sampled. This will incorrectly set the ingestion_reason span metadata to manual instead of rule (relevant doc).

Also, this doesn't cover the case when the parent span is not sampled. In that case, the child span shouldn't be sampled either. However, with this code, the child span will independently decide its sampling decision, so it might be sampled.