fix(warm-engine): answer replay queries under empty-schema deploy (was status=error)

[zzylol]

Summary

The MVP demo's replay.jsonl (/tmp/asap-mvp-rerun-postE/asap/measurements/replay.jsonl) showed the warm-tier engine returning status=error for nearly every replay query shape:

kind	replay query	rows	status
quantile	quantile_over_time(0.99, http_requests_total_latency_ms[1m])	686/686	error
count_unique	count(unique_users_per_min)	343/343	error
frequency	rate(endpoint_request_freq[5m])	342/342	error
topk	topk(5, top_endpoint_qps)	342/342	error
sum (instant)	sum by (zone) (http_requests_total)	343/343	error
sum (rate)	sum by (zone) (rate(http_requests_total[5m]))	0/343	success (cold tier)

PR #108's existing regression tests passed but did not mirror the production deploy shape — they seeded a populated PromQLSchema plus QueryConfig templates, so the engine never hit the schema-empty fallback paths the MVP harness exercises.

Root cause

The production warm-tier binary launches with --streaming-config=/etc/asap/streaming.yaml and no --config, so inference_config.schema = PromQLSchema::new() (empty) and inference_config.query_configs = []. Two engine paths blow up on this:

1. build_query_requirements_promql looked up the metric in inference_config.schema, fell back to KeyByLabelNames::empty() when missing, and built req.grouping_labels = []. The strict-equality labels_compatible then mismatched every controller-emitted agg config's [zone] grouping → capability miss → handle_query_promql returns None → format_unsupported_query_response writes status=error with http_code=200 (the exact replay.jsonl shape).

2. build_promql_execution_context_tail had its OWN schema lookup that returned None and short-circuited the whole query — even after capability matching had succeeded for queries like sum by (zone) (counter).

3. compatible_agg_types(Statistic::Count) did not list HLL, so count(<HLL-metric>) capability-missed even though HllSketchAccumulator answers Statistic::Count as a cardinality alias (hll_sketch_accumulator.rs:220).

4. compatible_agg_types(Statistic::Topk) did not list CountSketch, so topk(K, <CountSketch-metric>) capability-missed even though CountSketchAccumulator answers Statistic::Topk (count_sketch_accumulator.rs:284).

Fixes

• New SimpleEngine::resolve_metric_labels(metric) helper (engines/simple/engine.rs) that prefers the user-supplied schema and falls back to a deterministic union of grouping_labels across every StreamingConfig agg matching the metric. Both downstream call sites (build_query_requirements_promql, build_promql_execution_context_tail) now route through it.

• Relax labels_compatible (asap_types/src/capability_matching.rs) from strict-eq to subset (req ⊆ config). The TODO on this function asked for exactly this; the MVP demo's count(metric) / topk(K, metric) queries (no by (...) modifier) translate to req.grouping_labels = [] and must match a [zone] agg via the merge path. Direction is asymmetric: req ⊃ config is still rejected (engine cannot invent partitions).

• Add HLL to Statistic::Count compat list + add CountSketch to Statistic::Topk compat list.

• Tests: 5 new production_conditions_* tests in tests/datafusion/warm_engine_replay_regression_tests.rs reproducing each replay failure shape against an inference_config with empty PromQLSchema (mirroring the warm-tier deploy). 3 pass; 2 are #[ignore]'d as out-of-scope follow-ups (CountSketch needs paired SetAggregator under is_multi_population_value_type — PR harden(ingest): validate inbound CMS/CountSketch wire dimensions (defensive) #344 declares the capability on the controller side; CMS does not yet implement Statistic::Rate).

Test plan

• cargo test --release --lib -p query_engine_rust warm_engine_replay_regression_tests → 10 pass / 0 fail / 2 ignored (was 7 pass / 0 fail / 0 ignored on origin/main; 4 new prod-conditions tests, 3 pass, 1 ignored)
• cargo test --release --lib -p asap_types capability_matching → 38 pass / 0 fail (modulo a pre-existing flaky test avg_finds_sum_and_count that depends on HashMap iteration order, also flaky on origin/main)
• cargo test --release -p query_engine_rust --test inference_yaml_pattern_coverage → 11/11 pass (no regressions)
• cargo build --release → green
• Full cargo test --release --lib -p query_engine_rust failure set unchanged from origin/main (33 pre-existing failures, none related to this fix)
• Live curl evidence: not captured — running the MVP demo end-to-end requires rebuilding asap/query-backend:dev and re-running the soak harness; the unit tests pin the wire shape (expect("warm engine must answer …") against handle_query_promql).

Honest follow-ups (couldn't fix in one PR)

• topk(5, top_endpoint_qps): standalone CountSketch is is_multi_population_value_type and requires a paired SetAggregator/DeltaSetAggregator. The right structural fix is for the controller to plan top_endpoint_qps as CountMinSketchWithHeap (the integrated CMS+heap accumulator); PR harden(ingest): validate inbound CMS/CountSketch wire dimensions (defensive) #344 declares that capability on the controller side. Engine-side test marked #[ignore].
• rate(endpoint_request_freq[5m]): CountMinSketchAccumulator does not yet implement Statistic::Rate. Tracking via #[ignore]'d test + TODO note.
• sum_by_zone_instant_does_not_error flaky test (asap_types::capability_matching::tests::avg_finds_sum_and_count): pre-existing HashMap-ordering flake; should be stabilised by adding a tie-breaker to aggregation_priority. Out of scope for this PR.

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