chore: add test for merging baggage entries

[rachelyangdog]

What does this PR do?

Add a test to check that if baggage is set before a request is sent, it is merged into the baggage inside the request headers as well.

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.
• 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.
• For internal contributors, a matching PR should be created to the v2-dev branch and reviewed by @DataDog/apm-go.

Unsure? Have a question? Request a review!

[datadog-datadog-prod-us1]

Datadog Report

Branch report: rachel.yang/add-baggage-test
Commit report: 484813b
Test service: dd-trace-go

✅ 0 Failed, 5500 Passed, 72 Skipped, 10m 26.77s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-04-02 13:51:32

Comparing candidate commit 6809327 in PR branch rachel.yang/add-baggage-test with baseline commit d4109d5 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 61 metrics, 1 unstable metrics.

[rachelyangdog]

/merge

[dd-devflow]

View all feedbacks in Devflow UI.
2025-03-21 18:18:38 UTC ℹ️ Start processing command /merge

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2025-03-21 18:18:43 UTC ℹ️ MergeQueue: waiting for PR to be ready

This merge request is not mergeable yet, because of pending checks/missing approvals. It will be added to the queue as soon as checks pass and/or get approvals.
Note: if you pushed new commits since the last approval, you may need additional approval.
You can remove it from the waiting list with /remove command.

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2025-03-21 22:19:06 UTC ⚠️ MergeQueue: This merge request was unqueued

devflow unqueued this merge request: It did not become mergeable within the expected time