fix(metrics): skip V3 encoding for V2-only endpoints

[rayz]

Summary

• Determine whether any configured metrics endpoint requires V3 before constructing the V3 series encoder.
• Skip V3 serialization when every endpoint is using V2.
• Explicitly enable V3 series for the SMP blackhole endpoint so SMP exercises the actual V3 forwarding path instead of silently remaining on V2 because localhost does not match datadog_only.

This avoids encoding and buffering an unused V3 payload for V2-only configurations while retaining V3 encoding whenever at least one endpoint requires it.

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

How did you test this PR?

References

[datadog-official]

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🔗 Commit SHA: 0d1a573 | Docs | Datadog PR Page | Give us feedback!}

[pr-commenter]

Binary Size Analysis (Agent Data Plane)

Baseline: 032b9ac · Comparison: 0d1a573 · diff
Analysis Configuration: stripped binaries · Pass/Fail Threshold: +5%
Sizes: 40.83 MiB (baseline) vs 40.84 MiB (comparison)
Size Change: +5.55 KiB (+0.01%)

✅ Binary size difference within threshold

Changes by Module

Module	File Size	Symbols
hyper	+48.02 KiB	95
hyper_util	-43.39 KiB	10
figment	-40.84 KiB	185
saluki_components::common::datadog	+30.02 KiB	109
saluki_components::sources::otlp	+21.96 KiB	31
hashbrown	+18.42 KiB	158
axum	-17.96 KiB	116
std	-17.43 KiB	115
anon.1c766fa05dd13eba06da95c69824d6af.995.llvm.16845278109024732812	-17.36 KiB	1
anon.1c766fa05dd13eba06da95c69824d6af.995.llvm.2183139281678763661	+17.36 KiB	1
anon.1c5af32c575c0d14ae778d58023b5abf.0.llvm.10081054402879163701	+17.36 KiB	1
anon.ad88aefe8160b8dedfdf5305e59f030a.513.llvm.6948463008055664385	-17.27 KiB	1
saluki_components::encoders::datadog	+17.17 KiB	72
serde_core	+17.06 KiB	196
tokio_rustls	-12.72 KiB	12
chrono	-12.71 KiB	10
anon.ad88aefe8160b8dedfdf5305e59f030a.665.llvm.6948463008055664385	-12.20 KiB	1
anon.ffe0ccedd42a78dc35ad276479c57863.558.llvm.6176032040213260226	+12.02 KiB	1
crossbeam_channel	+10.30 KiB	31
serde	-10.25 KiB	24

Detailed Symbol Changes

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.7% +76.7Ki  +0.5% +47.2Ki    [15714 Others]
  [NEW] +59.7Ki  [NEW] +59.6Ki    saluki_components::common::datadog::io::run_endpoint_io_loop::_{{closure}}::h9313854c2f2a044f
  [NEW] +32.6Ki  [NEW] +32.3Ki    _<saluki_components::common::datadog::config::_::<impl serde_core::de::Deserialize for saluki_components::common::datadog::config::ForwarderConfiguration>::deserialize::__Visitor as serde_core::de::Visitor>::visit_map::h215858157e52a210
  [NEW] +17.4Ki  [NEW] +17.3Ki    anon.1c766fa05dd13eba06da95c69824d6af.995.llvm.2183139281678763661
  [NEW] +17.4Ki  [NEW]     +76    anon.1c5af32c575c0d14ae778d58023b5abf.0.llvm.10081054402879163701
  [NEW] +12.7Ki  [NEW] +12.5Ki    _<saluki_components::encoders::datadog::metrics::DatadogMetricsConfiguration as saluki_core::components::encoders::builder::EncoderBuilder>::build::_{{closure}}::h5ca37cf4fddb78c4
  [NEW] +12.1Ki  [NEW] +11.9Ki    _<figment::value::de::ConfiguredValueDe<I> as serde_core::de::Deserializer>::deserialize_struct::h2291bdd8b058a751
  [NEW] +12.0Ki  [NEW] +11.9Ki    _<T as alloc::string::SpecToString>::spec_to_string::h8144f32f6d7c3d68
  [DEL] -12.1Ki  [DEL] -11.9Ki    _<figment::value::magic::RelativePathBuf as figment::value::magic::Magic>::deserialize_from::h8bc594fc2cb35d4d
  [DEL] -12.2Ki  [DEL]     -81    anon.ad88aefe8160b8dedfdf5305e59f030a.665.llvm.6948463008055664385
  [DEL] -12.4Ki  [DEL] -12.3Ki    _<chrono::format::formatting::DelayedFormat<I> as core::fmt::Display>::fmt::h46581aaf02582bbc
 -74.3% -12.7Ki -74.6% -12.7Ki    prost::message::Message::encode::h28ad969e25341bd5
 -84.0% -13.7Ki -84.8% -13.7Ki    _<hyper_util::server::conn::auto::Connection<I,S,E> as core::future::future::Future>::poll::h5670434c8a709418
 -86.2% -13.8Ki -87.0% -13.8Ki    _<hyper_util::server::conn::auto::Connection<I,S,E> as core::future::future::Future>::poll::he89a2edaa7fcd54b
  [DEL] -14.3Ki  [DEL] -14.1Ki    _<figment::value::de::ConfiguredValueDe<I> as serde_core::de::Deserializer>::deserialize_any::h2e163da9da896fb8
 -85.3% -14.8Ki -86.1% -14.8Ki    _<hyper_util::server::conn::auto::Connection<I,S,E> as core::future::future::Future>::poll::hca913f7324cfdd5b
  [DEL] -14.9Ki  [DEL] -14.8Ki    _<figment::value::de::ConfiguredValueDe<I> as serde_core::de::Deserializer>::deserialize_struct::ha36113e815a9c1ca
  [DEL] -17.3Ki  [DEL]     -76    anon.ad88aefe8160b8dedfdf5305e59f030a.513.llvm.6948463008055664385
  [DEL] -17.4Ki  [DEL] -17.3Ki    anon.1c766fa05dd13eba06da95c69824d6af.995.llvm.16845278109024732812
  [DEL] -19.6Ki  [DEL] -19.5Ki    saluki_components::transforms::apm_stats::ApmStats::process_trace::h0c7467fe110ac0a8
  [DEL] -59.9Ki  [DEL] -59.7Ki    saluki_components::common::datadog::io::run_endpoint_io_loop::_{{closure}}::h3e35c9d992da4100
  +0.0% +5.55Ki  -0.0% -12.0Ki    TOTAL

[pr-commenter]

Regression Detector (Agent Data Plane)

Run ID: 937d188f-8545-475b-9267-1f4914ffa933
Baseline: 032b9ac5 · Comparison: 0d1a5731 · diff

Optimization Goals: ❌ 2 regressions detected

experiment	goal	Δ mean %	links
quality_gates_rss_dsd_heavy (erratic)	memory	🔴 +213.59	metrics profiles logs
quality_gates_rss_dsd_ultraheavy (erratic)	memory	🔴 +166.49	metrics profiles logs

Fine details of change detection per experiment (3)

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_medium (erratic)	memory	⚪ +3.13	metrics profiles logs
quality_gates_rss_dsd_low	memory	⚪ +0.07	metrics profiles logs
quality_gates_rss_idle	memory	⚪ -0.38	metrics profiles logs

Bounds Checks: ❌ Failed (5)

experiment	check	replicates	observed	links
quality_gates_rss_dsd_heavy	memory_usage	0/10	❌ 466 MiB ≤ 140 MiB	metrics profiles logs
quality_gates_rss_dsd_low	memory_usage	0/10	❌ 52.7 MiB ≤ 50 MiB	metrics profiles logs
quality_gates_rss_dsd_medium	memory_usage	0/10	❌ 96.6 MiB ≤ 75 MiB	metrics profiles logs
quality_gates_rss_dsd_ultraheavy	memory_usage	0/10	❌ 519 MiB ≤ 200 MiB	metrics profiles logs
quality_gates_rss_idle	memory_usage	10/10	✅ 31.5 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.

[tobz]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• fix(datadog): fix encoder deadlock under bursty v3 metrics #2229
• fix(metrics): skip V3 encoding for V2-only endpoints #2227 👈 (View in Graphite)
• chore(config): make Core Agent config authoritative for V3 routing #2226
• main

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