feat: per-tenant BackendStorageRouting emit (part 2/2)

[zzylol]

Summary

Pair with ASAPQuery-backend PR #107.
Adds an explicit tenant field to the controller-emitted
BackendStorageRouting JSON document so multi-tenant deploys can
push per-tenant routing tables to the backend's
POST /api/v1/storage_routing endpoint.

This is part 2/2 — the controller emit path. Part 1 (the
data model + per-tenant lookup + per-tenant hot-reload swap) has
already merged on ASAPQuery-backend@main.

Controller emit shape

• New emit_backend_storage_routing_for_tenant(tenant, plans)
  and emit_backend_storage_routing_with_prometheus_for_tenant( tenant, plans, mode3) entry points. The tenant lands in the
  emitted JSON's top-level tenant field, e.g.

{
  "tenant": "tenant-a",
  "default_engine": "sketch_warm_tier",
  "metrics": [ ... ]
}

• The single-tenant aliases (emit_backend_storage_routing /
  emit_backend_storage_routing_with_prometheus) delegate to
  the per-tenant variants with DEFAULT_TENANT = "default" —
  existing single-tenant main.rs callers remain unchanged.

Test coverage

• storage_routing_default_tenant_for_single_tenant_emit — the
  legacy entry point still emits tenant: "default".
• storage_routing_for_tenant_emits_explicit_tenant_field —
  the per-tenant entry point threads the explicit tenant id.
• storage_routing_with_prometheus_for_tenant_emits_explicit_tenant_field
  — same shape extends to the Mode-3 (Prometheus archive)
  emit path.
• storage_routing_three_metric_snapshot_stable (existing) —
  updated to include the tenant: "default" field at the end of
  the JSON (alphabetical key order in serde_json::Value::Map).

Out of scope (matches backend part 1/2)

• Tenant-aware AUTH.
• Per-tenant sketch state isolation.
• Threading a non-default tenant through the controller's
  workload registry / planner — main.rs still calls the
  single-tenant alias. Multi-tenant planning is a follow-up
  that needs the controller's workload registry to grow a
  tenant axis (separate from the routing-table follow-up).

Test plan

• cargo build --release (controller) passes
• cargo test --release — all stage_config tests green
  (42/42 stage-config; 10 pre-existing failures elsewhere
  on origin/main are unrelated to this change)
• Pairs with backend PR DEBS 2022 Case Study #107 (merged)

Related

• Issue MVP demo: test-first validation of ASAPCollector + ASAPQuery-backend #46 — multi-stage demo runbook
• PR mvp issue-46 ④ ⑥ partial — archive ground truth flowing; warm + probe routing remain #333 — left per-tenant scope as a follow-up
• ASAPQuery-backend PR DEBS 2022 Case Study #107 — data model + lookup + hot-reload

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