feat(trace sampler): emit sampler.seen/kept metrics

[thieman]

Summary

Implements all 6 sampler metrics from datadog-agent/pkg/trace/sampler/metrics.go and rare_sampler.go, part of #1134.

Metric	Type	Tags
datadog.trace_agent.sampler.seen	Counter	sampler, target_service, target_env¹, sampling_priority²
datadog.trace_agent.sampler.kept	Counter	same
datadog.trace_agent.sampler.size	Gauge	sampler (priority / no_priority / error)
datadog.trace_agent.sampler.rare.hits	Counter	none
datadog.trace_agent.sampler.rare.misses	Counter	none
datadog.trace_agent.sampler.rare.shrinks	Gauge	none

¹ target_env only emitted for priority, no_priority, rare, and error samplers
² sampling_priority only emitted for the priority sampler

Implementation details:

• run_samplers returns a SamplingDecision struct instead of a bare tuple, carrying sampler name and priority for metric recording
• seen/kept counter handles are lazily registered and cached in a FastHashMap — hot path pays only a hash-map lookup after the first observation of each tag combination
• rare.hits/rare.misses are incremented inline in RareSampler::sample(); rare.shrinks is updated each time SeenSpans::shrink() fires
• sampler.size gauges are updated after each transform_buffer batch by polling size() on each score-based sampler

Test plan

• All 43 existing trace sampler unit tests pass
• cargo clippy clean

🤖 Generated with Claude Code

[pr-commenter]

Binary Size Analysis (Agent Data Plane)

Target: bc3cc74 (baseline) vs 448ea06 (comparison) diff
Analysis Type: Stripped binaries (debug symbols excluded)
Baseline Size: 26.35 MiB
Comparison Size: 26.39 MiB
Size Change: +35.58 KiB (+0.13%)
Pass/Fail Threshold: +5%
Result: PASSED ✅

Changes by Module

Module	File Size	Symbols
saluki_components::transforms::trace_sampler	+15.28 KiB	8
core	+6.72 KiB	81
hashbrown	+6.41 KiB	4
[sections]	+4.06 KiB	5
saluki_metrics::builder::MetricsBuilder	+1.96 KiB	4
serde_core	+643 B	3
saluki_components::common::datadog	-607 B	23
saluki_components::encoders::datadog	+575 B	5
saluki_core::data_model::event	+352 B	1
http	+148 B	6
[Unmapped]	-105 B	1
unicode_segmentation	+88 B	1
saluki_components::transforms::dogstatsd_prefix_filter	+48 B	1
aho_corasick	+20 B	1
saluki_core::topology::blueprint	+9 B	1
tokio	+8 B	28
std	-4 B	15
figment	+3 B	1
agent_data_plane::cli::run	+2 B	2
saluki_common::task::instrument	+0 B	2

Detailed Symbol Changes

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +1.7% +26.4Ki  +2.1% +22.9Ki    [273 Others]
  [NEW] +25.6Ki  [NEW] +25.5Ki    saluki_components::transforms::trace_sampler::TraceSampler::process_trace::h6a6309045a0d5efa
  [NEW] +17.3Ki  [NEW] +17.2Ki    saluki_components::encoders::datadog::traces::TraceEndpointEncoder::encode_tracer_payload::_{{closure}}::_{{closure}}::h7f60427ea2bbc270
  [NEW] +13.2Ki  [NEW] +13.0Ki    saluki_components::common::datadog::request_builder::RequestBuilder<E>::encode_inner::_{{closure}}::h56c7675ff9353162
  [NEW] +12.4Ki  [NEW] +12.2Ki    _<core::marker::PhantomData<T> as serde_core::de::DeserializeSeed>::deserialize::h516ba3727ce3c3a6
  [NEW] +10.5Ki  [NEW] +10.3Ki    _<saluki_common::task::instrument::InstrumentedTask<F> as core::future::future::Future>::poll::h23e186975d91b998
  [NEW] +9.79Ki  [NEW] +9.67Ki    saluki_components::common::datadog::apm::ApmConfig::from_configuration::h06ca569aa0fe3ec0
  [NEW] +5.71Ki  [NEW] +5.55Ki    saluki_components::common::datadog::request_builder::RequestBuilder<E>::flush::_{{closure}}::h2da80e1539595dd3
  [NEW] +5.20Ki  [NEW] +5.05Ki    _<core::marker::PhantomData<T> as serde_core::de::DeserializeSeed>::deserialize::h5fabbb1c95dca95f
  [NEW] +3.83Ki  [NEW] +3.71Ki    <webpki::error::Error as core::fmt::Debug>::fmt.11262
  [NEW] +3.34Ki  [NEW] +3.17Ki    saluki_components::common::datadog::request_builder::RequestBuilder<E>::try_split_request::_{{closure}}::hfd70afba07daed71
  [DEL] -3.34Ki  [DEL] -3.18Ki    saluki_components::common::datadog::request_builder::RequestBuilder<E>::try_split_request::_{{closure}}::h3d6146a97391a71f
  [DEL] -3.83Ki  [DEL] -3.71Ki    <webpki::error::Error as core::fmt::Debug>::fmt.11256
  [DEL] -3.93Ki  [DEL] -3.78Ki    _<core::marker::PhantomData<T> as serde_core::de::DeserializeSeed>::deserialize::hde9fde945ac088d0
  [DEL] -5.72Ki  [DEL] -5.57Ki    saluki_components::common::datadog::request_builder::RequestBuilder<E>::flush::_{{closure}}::he10db712ee5679b7
  [DEL] -10.5Ki  [DEL] -10.3Ki    _<saluki_common::task::instrument::InstrumentedTask<F> as core::future::future::Future>::poll::hfbfad7a42c71c843
  [DEL] -11.0Ki  [DEL] -10.9Ki    saluki_components::common::datadog::apm::ApmConfig::from_configuration::hdcad4928457e0f0c
  [DEL] -12.5Ki  [DEL] -12.4Ki    _<core::marker::PhantomData<T> as serde_core::de::DeserializeSeed>::deserialize::habd4db8865bce350
  [DEL] -13.2Ki  [DEL] -13.1Ki    saluki_components::common::datadog::request_builder::RequestBuilder<E>::encode_inner::_{{closure}}::hcd0b9ca9428d1499
  [DEL] -16.3Ki  [DEL] -16.1Ki    _<saluki_components::transforms::trace_sampler::TraceSampler as saluki_core::components::transforms::SynchronousTransform>::transform_buffer::hf202555a35ab77d7
  [DEL] -17.3Ki  [DEL] -17.1Ki    saluki_components::encoders::datadog::traces::TraceEndpointEncoder::encode_tracer_payload::_{{closure}}::_{{closure}}::he82b1e6b98565373
  +0.1% +35.6Ki  +0.1% +32.1Ki    TOTAL

[pr-commenter]

Regression Detector (Agent Data Plane)

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