fix sfn x-datadog-tags input

[ojproductions]

What Does This Do

Builds the _datadog object injected into the Step Functions StartExecution input
using the standard context propagator instead of hand-written JSON.

The payload now reflects the configured propagation styles, and x-datadog-tags is
the encoded _dd.p.* string rather than a JSON object of the client span's tags.

Motivation

x-datadog-tags is defined as a string. Emitting it as an object caused the Datadog
Lambda extension's Step Functions deserializer to reject the whole _datadog payload,
so trace context was dropped across the Step Functions boundary. The object also never
carried the propagated tags downstream consumers rely on (_dd.p.tid, _dd.p.dm) —
it exposed unrelated span tags instead — and sampling priority was not injected at all.

Additional Notes

Consistent with the SNS and EventBridge instrumentations in this repo, and with how
dd-trace-js and dd-trace-py build the same _datadog payload.

Contributor Checklist

• Format the title according to the contribution guidelines
• Assign the type: and (comp: or inst:) labels in addition to any other useful labels
• Avoid using close, fix, or any linking keywords when referencing an issue
  Use solves instead, and assign the PR milestone to the issue
• Update the CODEOWNERS file on source file addition, migration, or deletion
• Update public documentation with any new configuration flags or behaviors
• Once approved, use merge queue to merge the PR

Jira ticket: SLES-2981

[ojproductions]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🎉

Reviewed commit: 3bb82d7eca

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[datadog-prod-us1-5]

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 69.74% (+10.95%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: e7f5aca | Docs | View more details | Give us feedback!}

[dd-octo-sts]

🟢 Java Benchmark SLOs — All performance SLOs passed

Suite	Status
Startup	🟢 pass

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	14.81 s	14.71 s	[-0.2%; +1.5%] (no difference)
startup:insecure-bank:tracing:Agent	13.67 s	13.76 s	[-1.6%; +0.4%] (no difference)
startup:petclinic:appsec:Agent	17.41 s	17.00 s	[+1.4%; +3.5%] (significantly worse)
startup:petclinic:iast:Agent	17.46 s	17.52 s	[-1.1%; +0.5%] (no difference)
startup:petclinic:profiling:Agent	17.32 s	17.33 s	[-1.3%; +1.2%] (no difference)
startup:petclinic:sca:Agent	16.74 s	17.35 s	[-7.7%; +0.7%] (no difference)
startup:petclinic:tracing:Agent	16.57 s	16.24 s	[-2.4%; +6.4%] (no difference)

Commit: e7f5aca7 · CI Pipeline · Benchmarking Platform UI

---

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

[datadog-prod-us1-5]

More details

The configured propagator writes the required sampling priority and string-valued propagation tags without a reportable regression.

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