Fix propagation tag serialization for trace chunks

[sarahchen6]

What Does This Do

Serialize the root span’s _dd.p.* propagation metadata on the first span of every V0.4, V0.5, and V1 protocol trace chunk

Motivation

Trace chunks that do not include the root lose the trace-level propagation tags as seen in the problem that #12092, which is specific to _dd.p.dm. Every chunk should carry this trace context exactly once.

Additional Notes

The changes in this PR resolve the root’s propagation tags directly, so the patch fix in #12092 would no longer be needed.

Contributor Checklist

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Jira ticket: [PROJ-IDENT]

[datadog-datadog-us1-prod]

🎯 Code Coverage (details)
• Patch Coverage: 85.19%
• Overall Coverage: 58.00% (+0.21%)

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[dd-octo-sts]

🟡 Java Benchmark SLOs — Performance SLO warning (near threshold)

Suite	Status
Startup	🟡 warning

SLO thresholds are defined here based on automatically generated metrics. A warning is raised when results are within 5% of the threshold.

PR vs. master results

Scenario	Candidate	master	Δ (95% CI of mean)
startup:insecure-bank:iast:Agent	13.94 s	13.96 s	[-0.8%; +0.6%] (no difference)
startup:insecure-bank:tracing:Agent	12.85 s	12.99 s	[-1.7%; -0.4%] (maybe better)
startup:petclinic:appsec:Agent	16.86 s	16.65 s	[+0.1%; +2.4%] (maybe worse)
startup:petclinic:iast:Agent	16.85 s	16.95 s	[-1.2%; +0.1%] (no difference)
startup:petclinic:profiling:Agent	16.77 s	16.86 s	[-1.8%; +0.6%] (no difference)
startup:petclinic:sca:Agent	16.86 s	16.54 s	[+0.8%; +3.0%] (maybe worse)
startup:petclinic:tracing:Agent	15.58 s	16.16 s	[-7.6%; +0.5%] (no difference)

Commit: c3e3deaa · CI Pipeline · Benchmarking Platform UI

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Load and DaCapo benchmarks can be triggered manually in the GitLab pipeline. Results will appear in the Benchmarking Platform UI after completion.

[dougqh]

Looks good to me. Thanks for digging into this.

[dd-octo-sts]

Hi! 👋 Thanks for your pull request! 🎉

To help us review it, please make sure to:

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[datadog-datadog-us1-prod]

More details

The chunk mappers now source propagation metadata from the local root while emitting it only on chunk index zero, and the surrounding call-site review found no behavioral regression or unhandled input shape. No additional tests recommended: the PR already adds coverage for root-first and partial-chunk serialization across all three protocols.

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[dd-octo-sts]

/merge

[gh-worker-devflow-routing-ef8351]

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2026-07-30 17:01:53 UTC ℹ️ Start processing command /merge

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2026-07-30 17:01:59 UTC ℹ️ MergeQueue: pull request added to the queue

The expected merge time in master is approximately 1h (p90).

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2026-07-30 17:51:57 UTC ℹ️ MergeQueue: This merge request was merged