refactor(propagation): Parse OpenTelemetry tracestate member

[MilanGarnier]

What Does This Do

Refactors W3C tracestate handling so the OpenTelemetry ot member is
represented separately while its value remains opaque.

• Extracts and re-emits the raw ot member.
• Preserves its inherited position when rebuilding tracestate.
• Does not interpret rv or th or connect the member to sampling.

Motivation

Isolate the tracestate parsing foundation from the consistent-sampling
behavior added by the follow-up PR.

Additional Notes

This is the first PR in a two-PR stack. The sampling behavior follows in
#12397.

Validation:

• :dd-trace-core:compileJava
• :dd-trace-core:spotlessCheck
• git diff --check

Contributor Checklist

• Format the title according to the contribution guidelines.
• Assign the required type: and comp: labels.
• Keep sampling behavior and tests in the follow-up PR.

Jira ticket: APMAPI-2171

[datadog-datadog-prod-us1]

🎯 Code Coverage (details)
• Patch Coverage: 52.43%
• Overall Coverage: 57.45% (-1.62%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 397a1ff | 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	13.96 s	14.02 s	[-1.5%; +0.5%] (no difference)
startup:insecure-bank:tracing:Agent	12.92 s	13.04 s	[-1.6%; -0.2%] (maybe better)
startup:petclinic:appsec:Agent	16.90 s	16.70 s	[+0.2%; +2.2%] (maybe worse)
startup:petclinic:iast:Agent	16.87 s	17.00 s	[-1.4%; -0.1%] (maybe better)
startup:petclinic:profiling:Agent	16.73 s	16.80 s	[-1.5%; +0.5%] (no difference)
startup:petclinic:sca:Agent	16.96 s	16.75 s	[+0.3%; +2.3%] (maybe worse)
startup:petclinic:tracing:Agent	16.01 s	16.22 s	[-2.5%; -0.1%] (maybe better)

Commit: 397a1ff6 · 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.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Reuse the parsed OTel state during reconciliation

perf: With the default DATADOG,TRACECONTEXT extraction order, matching Datadog and W3C headers cause fromHeaderValue to parse the tracestate before HttpCodec.applyTraceContextToFirstContext passes the same string here. Calling extractOtelTraceState then validates and scans it again—and allocates another substring/state when ot is present—on the inbound request path. Transfer the already-parsed state with the propagation tags instead of reparsing the raw header.

AGENTS.md reference: AGENTS.md:L76-L79

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[chatgpt-codex-connector]

Avoid counting the OTel member twice in the buffer estimate

perf: When the original tracestate length is already included above—either for a parsed W3CPTags containing dd or for a Datadog PTags updated during multi-style reconciliation—it already contains the ot= member and its delimiter. Adding its size again here over-allocates the StringBuilder backing storage by up to 260 characters on each fresh W3C encoding, including per-span encodings that use a last-parent override. Subtract the managed member from the original-size contribution or omit this second addition in those cases.

AGENTS.md reference: AGENTS.md:L76-L79

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[datadog-datadog-prod-us1]

More details

The change keeps the raw ot member and its inherited position when it rebuilds tracestate. The static review finds no reportable regression in the changed parsing and serialization paths.

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[mcculls]

The double-parsing issue will need to be fixed before this can be merged.

Claude suggests giving PTags a way to accept an already-built OtelTraceState directly (e.g. setOtelTraceState), and have applyTraceContextToFirstContext pass the parsed object from traceContext.getPropagationTags() instead of round-tripping through the string.

The double-counting issue is easily solved by removing the second addition.