fix(server): report stage-router stats for a composite route

[v0ropaev]

AlgorithmStats::new creates the stage-router baseline only when some route's algorithm name is exactly "stage_router":

stage_router_baseline: algorithms
    .contains(STAGE_ROUTER)
    .then(|| StageRouterCumulative::collect(&families)),

A composite route is a stage route under another name. CompositeRouter::new ends with

let route = build_stage_route(config.stage)?
    .with_name(COMPOSITE)
    .with_processor(Arc::new(setter));

and CompositeRouter::name() returns "composite", while the StageClassifier inside it calls record_score_metrics and record_routing_decision on every turn, with no dependence on the route name. So the counters reach /metrics and the JSON block does not: with no baseline the snapshot field stays None and #[serde(skip_serializing_if = "Option::is_none")] drops it, and GET /v1/stats answers "algorithm_stats":{}.

What the operator loses is the whole decision-source breakdown the stage-router documentation promises (docs/routing_algorithms/stage_router_routing.md:154, "/v1/stats reports decision counts by source and selected target, plus aggregate" scoring) for one of the documented strategies. subagents and auto are unaffected: the former delegates name() to its parent and the latter builds a real StageRouter.

The change keys the baseline on either name, next to the two names already hardcoded in that file.

Worth saying out loud: this is the narrow fix, not the structural one. The structural one is for libsy to say which algorithms a route composes, something like an Algorithm::composed_names() defaulting to [self.name()] and overridden by CompositeRouter, so that the next wrapper around build_stage_route does not reintroduce the gap. That touches a trait in another crate and is yours to want or not; say so and I will send it that way instead.

Tested: a_composite_route_reports_its_stage_router_stats, next to the existing projection test, driving StatsAccumulator::new(registry, ["composite"]) and the same instruments the existing test uses. It fails on main at the unwrap_or_else with "stage-router stats missing for a composite route". cargo test --workspace 926 passed, 0 failed. cargo fmt --all --check and cargo clippy -p switchyard-server --all-targets -- -D warnings clean on toolchain 1.99.

Not run: a live composite deployment. The test records the instruments directly rather than routing a turn through a composite route, because the recording path goes through the process-global OTel meter provider and that would make the test order-dependent. The half that is read rather than run is "a composite route records these instruments", which follows from CompositeRouter::new building the route with build_stage_route and StageClassifier::score recording unconditionally.

Summary by CodeRabbit

• Bug Fixes
  • Stats snapshots for composite routes now include stage-router routing metrics, including routing decisions.

[coderabbitai]

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ℹ️ Recent review info

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• Configuration used: Repository: NVIDIA-NeMo/Switchyard/.coderabbit.yaml
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• Plan: Enterprise
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📥 Commits

Reviewing files that changed from the base of the PR and between a3cdc51 and 4098e31.

📒 Files selected for processing (2)

• crates/switchyard-server/src/stats/algorithms.rs
• crates/switchyard-server/src/stats/algorithms/stage_router.rs

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Walkthrough

Composite routes now include stage-router metrics in stats snapshots. A new test checks that a composite route reports three recorded routing decisions for llm-classifier.

Changes

Composite Route Stats

Layer / File(s)	Summary
Collect stage-router metrics for composite routes crates/switchyard-server/src/stats/algorithms.rs, crates/switchyard-server/src/stats/algorithms/stage_router.rs	The stats accumulator collects the stage-router baseline for composite as well as stage_router. A test verifies that a composite-route snapshot reports three llm-classifier routing decisions.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 4098e

Composite routes now have the metric baseline needed to report their stage-router decisions in /v1/stats. No actionable merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: reporting stage-router statistics for composite routes.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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---

_{A rabbit counts the routes in spring,
Three choices hop through stats to sing.
Composite joins the stage-router trail,
Its snapshot keeps each count in scale.
I tuck the numbers in my den,
Then check the total: three again!}

_{Comment @coderabbitai help to get the list of available commands.}