feat(trace-sampler): apply APM_SAMPLING remote configurations

[webern]

Human Summary

Adds a TraceSamplingSubscription and decoder for its remote configuration protocol.

AI Summary

The RC client in #2713 can deliver typed product snapshots, and #2719 handles semantic mappings for trace translation and stats. This PR adds the separate APM_SAMPLING consumer: its decoder interprets remote sampler settings, and the trace sampler can apply accepted settings while processing traces. The sampler configuration takes a TraceSamplingSubscription; without one, it holds an inert subscription and continues using its configured settings. #2724 is responsible for constructing the production subscription and passing it into the sampler.

APM_SAMPLING can override the priority and errors sampler target traces per second and enable or disable rare sampling. For each field, the sampler takes the last matching by_env entry, then all_envs, then its static setting. Missing or null fields leave that field unset; an explicit zero or false is an override. The no-priority sampler target is not remotely changed.

• SamplingDecoder accepts one configuration. It rejects malformed input or multiple assigned configurations. With none assigned, it publishes an Unassigned snapshot: unlike the semantic registry, the sampler keeps its last applied remote settings instead of returning to static settings.
• The sampler reads its latest accepted snapshot before each input event buffer, applies a new one once, then processes that buffer with those settings. Rejected updates leave its last applied settings in place.
• Tests cover decoding, environment precedence, and updates through a built sampler. Without feat(adp): apply trace products from remote configuration #2724's startup wiring, production still uses static sampler settings.

Change Type

• New feature

How did you test this PR?

• make fmt, cargo check --workspace, cargo check --workspace --tests, make check-docs, make check-clippy at the stack tip.
• Filtered saluki-components tests: 549 passed at the stack tip.

References

• Progresses: Traces(otlp+dd): Apply APM sampling updates from remote config #2434
• Related: feat(remote-config): add a remote configuration client #2713

[pr-commenter]

Binary Size Analysis (Agent Data Plane)

Baseline: 216e499 · Comparison: effc81a · diff
Analysis Configuration: stripped binaries · Pass/Fail Threshold: +5%
Sizes: 38.27 MiB (baseline) vs 38.29 MiB (comparison)
Size Change: +26.43 KiB (+0.07%)

✅ Binary size difference within threshold

Changes by Module

Module	File Size	Symbols
core	+60.47 KiB	8993
&mut serde_json	-35.73 KiB	91
agent_data_plane::state::metrics	-35.28 KiB	6
agent_data_plane::internal::remote_agent	+24.68 KiB	118
otlp_protos::otlp_include::opentelemetry	-18.26 KiB	181
anon.fae0c96d29a9fe1386d81498a79beb50.30.llvm.3370098010730192185	+16.19 KiB	1
anon.87e7c38bfd928d17fa28187081c51bf0.2.llvm.240572210426274404	-16.09 KiB	1
saluki_components::sources::otlp	+14.89 KiB	259
anon.fae0c96d29a9fe1386d81498a79beb50.458.llvm.3370098010730192185	+14.63 KiB	1
anon.87e7c38bfd928d17fa28187081c51bf0.768.llvm.240572210426274404	-14.62 KiB	1
serde_json	+14.26 KiB	453
anon.fae0c96d29a9fe1386d81498a79beb50.6.llvm.3370098010730192185	+12.84 KiB	1
anon.87e7c38bfd928d17fa28187081c51bf0.766.llvm.240572210426274404	-12.84 KiB	1
[sections]	-12.76 KiB	9
piecemeal	-11.67 KiB	36
anon.9d1057ec6821611763596b0232be6ef0.693.llvm.6678353967787269721	-11.13 KiB	1
saluki_core::cache::CacheBuilder<K,V,W,H>	+11.03 KiB	6
agent_data_plane::internal::telemetry	+10.92 KiB	8
anon.3716a808dbcf00fa1ff5dbde2056a4bd.671.llvm.9155267978429562090	+10.87 KiB	1
saluki_components::sources::dogstatsd	+10.05 KiB	397

Detailed Symbol Changes

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  [NEW] +59.8Ki  [NEW] +59.7Ki    saluki_components::common::datadog::io::run_endpoint_io_loop::_{{closure}}::ha38a49b72e023545
  [NEW] +43.5Ki  [NEW] +43.3Ki    _<saluki_components::forwarders::otlp::OtlpForwarder as saluki_core::components::forwarders::Forwarder>::run::_{{closure}}::hb872765f165820b0
  [NEW] +40.6Ki  [NEW] +40.5Ki    agent_data_plane::cli::run::create_topology::_{{closure}}::h75562c7ea09bcbff
  [NEW] +37.1Ki  [NEW] +36.9Ki    _<saluki_components::transforms::apm_stats::ApmStats as saluki_core::components::transforms::Transform>::run::_{{closure}}::h79ca62828ea1b270
  [NEW] +34.4Ki  [NEW] +34.2Ki    agent_data_plane::cli::run::handle_run_command::_{{closure}}::h0f7a307e9cc0a16a
  [NEW] +30.8Ki  [NEW] +30.7Ki    agent_data_plane::cli::dogstatsd::run_dogstatsd_command::_{{closure}}::hdfcd4810bc0a3053
  [NEW] +28.7Ki  [NEW] +28.6Ki    agent_data_plane::dogstatsd_contexts::artifact::for_each_record::h1b4e32864ddc5f08
  [NEW] +27.7Ki  [NEW] +27.5Ki    datadog_agent_commons::ipc::client::RemoteAgentClient::connect::_{{closure}}::_{{closure}}::_{{closure}}::h0a198a9356f00b58
  +0.2% +27.6Ki  +0.1% +9.17Ki    [28931 Others]
  [NEW] +25.5Ki  [NEW] +25.4Ki    agent_data_plane::internal::remote_agent::run_remote_agent_registration_loop::_{{closure}}::h044bf8d66771780c
  [NEW] +25.1Ki  [NEW] +25.0Ki    agent_data_plane::internal::remote_agent::RemoteAgentBootstrap::build_impl::h9676872f68a8b683
  [DEL] -25.6Ki  [DEL] -25.4Ki    agent_data_plane::internal::remote_agent::run_remote_agent_registration_loop::_{{closure}}::h1f55b7becb8f9961
  [DEL] -25.7Ki  [DEL] -25.6Ki    core::ops::function::FnOnce::call_once::h4d6ad556f476b8c9
  [DEL] -28.4Ki  [DEL] -28.2Ki    datadog_agent_commons::ipc::client::RemoteAgentClient::connect::_{{closure}}::_{{closure}}::_{{closure}}::h1bd6a026eeafe819
  [DEL] -28.7Ki  [DEL] -28.6Ki    agent_data_plane::dogstatsd_contexts::artifact::for_each_record::hb60d5262ffff75a0
  [DEL] -31.4Ki  [DEL] -31.2Ki    agent_data_plane::cli::dogstatsd::run_dogstatsd_command::_{{closure}}::h2f5246c806e7f392
  [DEL] -34.9Ki  [DEL] -34.7Ki    agent_data_plane::cli::run::handle_run_command::_{{closure}}::haa79196f741be08a
  [DEL] -35.8Ki  [DEL] -35.6Ki    _<saluki_components::transforms::apm_stats::ApmStats as saluki_core::components::transforms::Transform>::run::_{{closure}}::h8bffda5d06bacbfd
  [DEL] -40.5Ki  [DEL] -40.4Ki    agent_data_plane::cli::run::create_topology::_{{closure}}::h02ed82f16b54dbc0
  [DEL] -44.7Ki  [DEL] -44.5Ki    _<saluki_components::forwarders::otlp::OtlpForwarder as saluki_core::components::forwarders::Forwarder>::run::_{{closure}}::h252ca254b106e4c3
  [DEL] -58.6Ki  [DEL] -58.5Ki    saluki_components::common::datadog::io::run_endpoint_io_loop::_{{closure}}::h15a965fc1b5a6f1e
  +0.1% +26.4Ki  +0.0% +7.96Ki    TOTAL

[pr-commenter]

Regression Detector (Agent Data Plane)

Run ID: f8722aa1-4b9b-48f4-b1d3-4659383bd4b2
Baseline: 216e499a · Comparison: effc81af · diff

Optimization Goals: ✅ No significant changes detected

Fine details of change detection per experiment (5)

Experiments configured erratic: true are tagged (ignored) and skipped when determining which experiments regressed or improved. Experiments which are detected as erratic at runtime are tagged (erratic) to flag that the run's sample dispersion was high, but their regression / improvement signal still counts.

experiment	goal	Δ mean %	links
quality_gates_rss_dsd_heavy	memory	⚪ -0.22	metrics profiles logs
quality_gates_rss_dsd_medium	memory	⚪ -0.55	metrics profiles logs
quality_gates_rss_idle	memory	⚪ -0.67	metrics profiles logs
quality_gates_rss_dsd_low	memory	⚪ -0.84	metrics profiles logs
quality_gates_rss_dsd_ultraheavy	memory	⚪ -1.23	metrics profiles logs

Bounds Checks: ✅ Passed (5)

experiment	check	replicates	observed	links
quality_gates_rss_dsd_heavy	memory_usage	10/10	✅ 229 MiB ≤ 250 MiB	metrics profiles logs
quality_gates_rss_dsd_low	memory_usage	10/10	✅ 51.4 MiB ≤ 60 MiB	metrics profiles logs
quality_gates_rss_dsd_medium	memory_usage	10/10	✅ 90.6 MiB ≤ 100 MiB	metrics profiles logs
quality_gates_rss_dsd_ultraheavy	memory_usage	10/10	✅ 385 MiB ≤ 420 MiB	metrics profiles logs
quality_gates_rss_idle	memory_usage	10/10	✅ 33 MiB ≤ 40 MiB	metrics profiles logs

Explanation

A change is flagged as a regression when |Δ mean %| > 5.00% in the regressing direction for its optimization goal AND SMP marks the experiment as a regression (is_regression: true). Improvements use the matching criteria for the improving direction. Experiments configured erratic: true (tagged (ignored)) are skipped outright; experiments detected as erratic at runtime (tagged (erratic)) still count, since that flag describes sample dispersion rather than directional certainty. The Δ mean % cell is colored accordingly: 🟢 = improvement, 🔴 = regression, ⚪ = neutral. Reduction in CPU or memory is an improvement; reduction in ingress throughput is a regression. Experiments tagged (no analysis) show ⚠️ n/a: SMP ran them but produced no analysis, usually because a replicate failed and exhausted its retries. Check the SMP report for that experiment's replicate failures.

[chatgpt-codex-connector]

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[datadog-datadog-prod-us1-2]

A remote target restored from zero within one sampling bucket leaves learned rates at zero until the next recalculation, temporarily suppressing error traces despite a positive effective target.

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