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 a
span carries ai_guard.event=true, the sampler is skipped (whether that sampler
is the regular priority sampler or the AsmStandaloneSampler rate-limiter used
in ASM standalone mode).

The ai_guard.event tag is also promoted from a local string constant in
AIGuardInternal to a canonical constant in Tags, alongside ai_guard.keep.

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
• 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.

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  • Get more information in this doc

Jira ticket: APPSEC-68487

[claponcet]

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[datadog-prod-us1-5]

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 56.58% (-0.46%)

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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	13.98 s	13.97 s	[-0.5%; +0.6%] (no difference)
startup:insecure-bank:tracing:Agent	12.94 s	13.04 s	[-1.5%; +0.0%] (no difference)
startup:petclinic:appsec:Agent	16.83 s	16.64 s	[+0.2%; +2.1%] (maybe worse)
startup:petclinic:iast:Agent	16.78 s	16.96 s	[-1.9%; -0.2%] (maybe better)
startup:petclinic:profiling:Agent	16.43 s	16.70 s	[-6.2%; +3.0%] (no difference)
startup:petclinic:sca:Agent	16.91 s	16.87 s	[-0.7%; +1.2%] (no difference)
startup:petclinic:tracing:Agent	16.05 s	16.39 s	[-3.1%; -1.1%] (significantly better)

Commit: 2490539f · 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.