mvp phase α: controller emits BackendStorageRouting via stage_config

[zzylol]

Summary

Phase α of the planner consolidation: the controller becomes the sole authority for BackendStorageRouting. Today the routing table is hand-authored YAML at deploy/configs/backend-storage-routing.yaml; after Phase α the controller emits it from its typed L5 stage_config and pushes it to the backend on every plan emit.

• config::stage_config::emit_backend_storage_routing(&[(metric, &BackendStageConfig)]) — derives (metric, [target]) rows from the L4 sketch families landed at the backend. Classification rules:
  • DDSketch / KLL planned → warm tier serves quantile / quantile_over_time.
  • HLL planned → warm tier serves count (cardinality readout). count drops off the archive's claim list.
  • Count-Sketch planned → warm tier serves topk (heap-augmented Estimate). topk drops off the archive's claim list.
  • CMS planned → warm tier serves count (CMS Estimate readout).
  • Archive-eligible shapes — histogram_quantile, delta, deriv, absent, rate_post_hoc, plus topk / count when no matching sketch was planned.
  • Warm-tier slot stays the default (no applies_to_query_shape) — unanticipated shapes route to warm rather than falling through to an archive first-target fallback.
• BackendClient::post_storage_routing_json — sibling of post_streaming_config_json. Rewrites the streaming-config URL's path to /api/v1/storage_routing so operators configure one CONTROLLER_BACKEND_ENDPOINT.
• main::handle_plan — when USE_TYPED_STAGE_SPLIT=1, also emit + POST the routing JSON. Same fire-and-forget contract as the existing streaming-config push.

Test plan

• cargo build clean
• 11 new tests pass (7 emitter unit / snapshot tests, 2 URL-derivation tests, 2 mock-backend integration tests)
• No regression in existing tests (493 pass, 10 fail — same 10 pre-existing failures as origin/main)
• Snapshot test pins the exact JSON shape for a DDSketch + HLL + Count-Sketch plan

Schema example

The snapshot test produces (for three metrics, mix of sketch families):

```json
{
"default_engine": "sketch_warm_tier",
"metrics": [
{
"name": "http_requests_total",
"targets": [
{ "engine": "sketch_warm_tier" },
{
"engine": "thanos_archive",
"applies_to_query_shape": [
"histogram_quantile", "delta", "deriv",
"absent", "rate_post_hoc", "count"
]
}
],
"warm_tier_native_shapes": ["topk", "rate", "sum", "avg", "min", "max"]
}
]
}
```

Coordination

• Behind `USE_TYPED_STAGE_SPLIT=1` flag — safe to merge ahead of the backend consumer (Part B, separate PR).
• Phase β (asap-planner-rs pattern migration) does not conflict — Phase α only extends the existing emitter.
• Auto-merge if green.

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