mvp phase ε.1: three-mode cost model with native OTLP to Prometheus (Mode 3)

[zzylol]

Summary

Phase ε.1 collapses the planner's raw-vs-sketch + edge-vs-backend axes
into a single tri-mode BindMode selector. All three modes use OTLP
on the wire — Mode 3 ships raw OTLP directly to Prometheus's native
OTLP receiver at /api/v1/otlp/v1/metrics (Prom 2.47+, activated with
--web.enable-otlp-receiver).

Three modes:

• Mode 1 (SketchAtEdge) — sketch processor at edge ships sketch state
  (default for high samples_per_window).
• Mode 2 (RawAtEdge_SketchAtBackend) — raw OTLP at edge, backend
  builds sketch at ingest (when edge_cpu_budget is tight). Phase ε.2
  implements the raw-input ingest path.
• Mode 3 (RawAtEdge_PrometheusArchive) — raw OTLP straight to
  Prometheus; backend HTTP-forwards queries to /api/v1/query. Phase
  ε.2 wires the prometheus_remote engine on the backend.

Changes

• planner::wire_cost — DELTA-encoded per-sketch wire-cost table,
  break-even-samples math, three-mode BindMode with the Phase-ε.1
  selection rules.
• SketchExpr::RawAtEdgeSketchAtBackend and
  SketchExpr::RawAtEdgePrometheusArchive variants + allocator /
  emitter walks.
• BackendAggregation::aggregation_input (sketch_envelope vs raw) and
  EdgeStageConfig::prometheus_archive_metrics.
• emit_edge_yaml adds an otlphttp/prometheus exporter +
  routing processor when Mode 3 metrics are present.
• emit_backend_storage_routing_with_prometheus emits
  engine: prometheus_remote for Mode 3 metrics.
• deploy/configs/prometheus-otlp-receiver.yml and
  deploy/docker-compose/mvp-prometheus-archive.yml for the deploy
  side.

Break-even table (50 B per raw sample)

Sketch family	State (B)	Break-even (samples/window)
DDSketch+delta	600	16
KLL (full)	3,000	64
HLL+delta	10,000	204
Count-Min+delta	4,000	84
CountSketch+delta	250,000	5,004

KLL has no delta variant (per Implementation.tex — randomised
compaction is not additively mergeable).

Test plan

• cargo test --release -p controller — 533 passed (vs. 516 baseline =
  17 new tests pass), 10 pre-existing failures unchanged.
• All wire_cost / stage_config Phase ε.1 tests green
  (cargo test --release phase_eps1 → 6/6 ok;
  cargo test --release planner::wire_cost → 11/11 ok).
• Step-2.4 demo runs verifying Mode 3 e2e against a live Prometheus
  with --web.enable-otlp-receiver (separate PR).

Notes for Phase ε.2

• Engine ID: prometheus_remote.
• Query URL: ${ASAP_PROMETHEUS_QUERY_URL:-http://prometheus-archive:9090}/api/v1/query.
• Expected forwarder shape: pass-through of PromQL string + time (or
  start/end/step for range queries) → Prometheus → re-emit JSON
  result back to the backend's HTTP query handler.
• BackendAggregation::aggregation_input == Raw requires an OTLP-raw
  ingest path on OtlpReceiver that builds the sketch from samples on
  arrival (Mode 2; out of scope for ε.1).

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