Honour request header tags mapped onto headers

[ValentinZakharov]

What Does This Do

Makes a DD_TRACE_REQUEST_HEADER_TAGS mapping on a header that the tracer also consumes for another purpose actually produce its tag.

Every context interpreter recognises certain headers - User-Agent, the Forwarded / X-Forwarded-* family, and the client-IP headers (X-Real-IP, True-Client-IP, CF-Connecting-IP, the configurable DD_TRACE_CLIENT_IP_HEADER, …) - captures them via the handled* helpers, and then returns from accept() immediately. That early return means the header never reaches handleTags(), so a header tag mapped onto it is silently dropped.

This is the same class of bug that #12662 fixed for X-Amzn-Trace-Id, generalised to the remaining consumed headers. Instead of removing each early return one by one, this PR applies the header→tag / header→baggage mappings once up front in accept(), so no later early return can bypass them. The semantic capture of each header (IP, user-agent, forwarding, trace context) is unchanged.

The fix is applied to every interpreter that carries this pattern: DatadogHttpCodec, XRayHttpCodec, W3CHttpCodec, HaystackHttpCodec, B3HttpCodec (both multi and single) and NoneCodec. Each interpreter keeps its existing tag/baggage semantics; the mappings simply run before the switch rather than after it.

Motivation

APMS-20654

Additional Notes

Because the mappings now run before the switch, a mapping onto a header that was previously consumed and returned early - including the trace-context headers themselves - is now honoured, consistent with every other header. X-Amzn-Trace-Id is covered as a side effect, so this supersedes #12662 if merged first.

ConsumedHeaderTagMappingTest exercises every consumed header across all seven interpreters and asserts that the mapped tag is produced, the semantic capture is preserved, and the trace context is still extracted. All new tests were confirmed to fail without the codec changes.

Contributor Checklist

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Jira ticket: [PROJ-IDENT]

[datadog-prod-us1-4]

More details

Moving tag and baggage mapping ahead of consumed-header short circuits preserves existing precedence and semantic extraction while allowing configured mappings to take effect.

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🟢 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	14.08 s	13.96 s	[+0.1%; +1.6%] (maybe worse)
startup:insecure-bank:tracing:Agent	12.89 s	12.96 s	[-1.3%; +0.1%] (no difference)
startup:petclinic:appsec:Agent	17.00 s	16.92 s	[-0.5%; +1.4%] (no difference)
startup:petclinic:iast:Agent	16.91 s	17.03 s	[-1.8%; +0.4%] (no difference)
startup:petclinic:profiling:Agent	16.25 s	16.72 s	[-7.0%; +1.3%] (no difference)
startup:petclinic:sca:Agent	16.99 s	16.76 s	[+0.6%; +2.1%] (maybe worse)
startup:petclinic:tracing:Agent	16.17 s	16.28 s	[-1.7%; +0.4%] (no difference)

Commit: 483781b0 · CI Pipeline · Benchmarking Platform UI

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