Implement DD_APM_TRACING_ENABLED=false

[cataphract]

Description

Implement DD_APM_TRACING_ENABLED=false, as described in the relevant RFC ("Configuration to Disable APM Tracing"). It also tries to keep at least about roughly 1 trace/second, so the application is deemed active.

[Copilot]

Pull Request Overview

This PR introduces a mechanism to disable APM tracing via DD_APM_TRACING_ENABLED=false, while allowing at least one trace per second to be sampled when disabled, and propagates the new config flag through agent, tracer, and telemetry components.

• Added ApmDisabledTraceSampler for rate-limited sampling when tracing is disabled
• Introduced apm_tracing_enabled flag across config loading, agent, tracer, and telemetry
• Switched to ErasedTraceSampler to unify sampler interfaces

Reviewed Changes

Copilot reviewed 30 out of 30 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/datadog/trace_sampler.cpp	Implemented ApmDisabledTraceSampler, updated erasure model
src/datadog/telemetry/telemetry_impl.cpp	Added APM_TRACING_ENABLED serialization in telemetry names
src/datadog/tags.{h,cpp}	Declared _dd.p.ts and _dd.apm.enabled internal tags
src/datadog/extraction_util.cpp	Extended trace tag parsing for _dd.p.ts
src/datadog/datadog_agent_config.{h,cpp}	Loaded and finalized apm_tracing_enabled from env/user
src/datadog/datadog_agent.{h,cpp}	Stored flag on agent and conditionally set HTTP header
src/datadog/config_manager.{h,cpp}	Wrapped TraceSampler in ErasedTraceSampler
include/datadog/{tracer_config,tracer}.h	Exposed APM tracing flag and disabled sampler in tracer API
include/datadog/trace_segment.h	Added member apm_tracing_enabled_ to TraceSegment
include/datadog/{collector,config,environment}.h	Updated enums and environment lookup for new flag
include/datadog/sampling_mechanism.h	Updated enum comment for APP_SEC mechanism
include/datadog/injection_options.h	Added has_appsec_matches field

Comments suppressed due to low confidence (2)

include/datadog/tracer.h:42

• [nitpick] The field name apm_tracing_disabled_sampler_ is potentially confusing; consider renaming to something like apm_disabled_sampler_ or disabled_trace_sampler_ to clarify its intent.

std::shared_ptr<ErasedTraceSampler> apm_tracing_disabled_sampler_;

src/datadog/trace_sampler.cpp:128

• Consider adding unit tests for ApmDisabledTraceSampler to verify rate-limiting behavior and ensure that at least one trace per second is retained when tracing is disabled.

SamplingDecision ApmDisabledTraceSampler::decide(const SpanData& span_data) {