Support DD_APM_TRACING_ENABLED=false for AI Guard traces

[claponcet]

What Does This Do

When DD_APM_TRACING_ENABLED=false, AI Guard traces are now preserved and sent
to the backend. The tracer no longer lets the sampler override the USER_KEEP
priority that AI Guard sets via ai_guard.keep.

Concretely, TraceCollector.setSamplingPriorityIfNecessary gains a guard: if
the root span's propagated trace source (_dd.p.ts) is marked for AI_GUARD
(new ProductTraceSource.AI_GUARD bit), the sampler is skipped entirely
(whether that sampler is the regular priority sampler or the
AsmStandaloneSampler rate-limiter used in ASM standalone mode) — mirroring
the existing bypass already in place for ASM.

AIGuardInternal now sets Tags.PROPAGATED_TRACE_SOURCE (_dd.p.ts) to
ProductTraceSource.AI_GUARD on the local root span alongside ai_guard.keep,
so the trace source itself signals "do not let the sampler touch this."

The ai_guard.event tag is also promoted from a local string constant in
AIGuardInternal to a canonical constant in Tags (Tags.AI_GUARD_EVENT),
alongside ai_guard.keep. This tag is purely descriptive (marks a span as
originating from an AI Guard evaluation) and plays no role in the
sampler-bypass logic.

Root spans for AI Guard evaluations carry:

• _sampling_priority_v1:2 (USER_KEEP) — set by forceKeep(SamplingMechanism.AI_GUARD)
• _dd.p.dm:-13 (decision maker: AI Guard) — set by the same mechanism
• _dd.p.ts marked with the AI_GUARD bit — set via Tags.PROPAGATED_TRACE_SOURCE, causes the sampler to be skipped
• ai_guard.event:true

Motivation

Part of APPSEC-61460 (AI Guard without APM billing). AI Guard customers should
not require APM host billing. Unlike ASM standalone mode, AI Guard does not need
the "1 trace per minute" allowance — every AI Guard trace must go through.

Additional Notes

The AsmStandaloneSampler (used when APM is disabled and AppSec is enabled)
has a rate limiter that allows only 1 trace/minute for regular APM traces. The
new bypass ensures AI Guard traces are never dropped by this limiter, while
regular traces remain subject to it. The bypass is keyed off the propagated
trace source (_dd.p.ts) rather than the ai_guard.event tag, so it composes
with the existing ASM bypass via ProductTraceSource.isProductMarked(..., ASM, AI_GUARD).

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Jira ticket: APPSEC-68487

[claponcet]

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[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	13.92 s	14.01 s	[-1.4%; +0.1%] (no difference)
startup:insecure-bank:tracing:Agent	12.88 s	12.97 s	[-1.4%; +0.0%] (no difference)
startup:petclinic:appsec:Agent	16.94 s	16.61 s	[+1.0%; +2.9%] (significantly worse)
startup:petclinic:iast:Agent	16.94 s	16.98 s	[-1.1%; +0.6%] (no difference)
startup:petclinic:profiling:Agent	16.81 s	16.88 s	[-1.6%; +0.8%] (no difference)
startup:petclinic:sca:Agent	16.95 s	16.83 s	[-0.2%; +1.6%] (no difference)
startup:petclinic:tracing:Agent	16.11 s	16.19 s	[-1.5%; +0.4%] (no difference)

Commit: 5c40eea3 · 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.

[jandro996]

One minor non-blocking nit inline about varargs allocation in isProductMarked.
No other issues found

[dd-octo-sts]

/merge

[gh-worker-devflow-routing-ef8351]

View all feedbacks in Devflow UI.

2026-07-22 09:56:32 UTC ℹ️ Start processing command /merge

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2026-07-22 09:56:37 UTC ℹ️ MergeQueue: pull request added to the queue

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

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2026-07-22 10:54:57 UTC ℹ️ MergeQueue: This merge request was merged