Map bridge/2DC pool topology in @ydbjs/telemetry

[polRk]

What

Wire the bridge (2DC) topology diagnostics_channel events that @ydbjs/core now publishes into @ydbjs/telemetry OTel spans and metrics.

Why

Core's endpoints engine gained bridge/pool observability (8f0fc0f), but @ydbjs/telemetry only mapped the ydb.node.pile connection attribute. The new discovery.completed fields (selfLocation, piles, primaryPile) and the new pool.opened / pool.stats / pile.changed / pile.fallback channels went unconsumed, so bridge topology and the aggregate routing snapshot were invisible in traces and metrics. This completes the mapping.

Changes

• Traces (channels.ts, semconv/{spans,events}.ts)
  • ydb:driver.discovery.completed also sets ydb.discovery.self_location and ydb.discovery.primary_pile on the ydb.Discovery span (dropped off-bridge).
  • New event mapping: ydb:driver.pile.changed → ydb.driver.pile.changed span event with ydb.driver.pile.primary_before/primary_after. It fires inside the discovery span (in publishRoundDiagnostics), so it attaches; the structured roster stays on the bus.
• Metrics (metrics.ts, state/connection-pool.ts, semconv/metrics.ts)
  • Observable gauges from ydb:driver.connection.pool.stats, keyed by DriverIdentity: ydb.driver.pool.routable (tag ydb.routing.tier ∈ {prefer, fallback}), ydb.driver.pool.pessimized, ydb.driver.pool.nodes (per pile, tagged ydb.pile.name/ydb.pile.status).
  • ydb:driver.connection.pool.opened config folded into per-driver state; preferPrimaryPile/localityEnabled ride the routable gauge as tags (they define what prefer/fallback mean; one fixed value per driver → bounded cardinality).
  • Counters ydb.driver.pile.fallbacks (tagged ydb.pile.fallback.active) and ydb.driver.pile.changes. State cleared on ydb:driver.closed.
  • pool.stats / pile.fallback fire outside any span (async consume loop) → metrics-only, never attached to a span, per the producer contract.
• Docs: extend the telemetry changeset (bumped patch → minor — new instruments/attributes are a backwards-compatible feature) and the README channel/instrument tables + cardinality budget.

No breaking changes — purely additive.

Testing

• New packages/telemetry/src/bridge.test.ts (10 tests): span-event assertions (discovery attrs bridge + non-bridge, pile.changed event) and metric-reader assertions (all three pool gauges, routing-mode tags present/absent, both counters, teardown on driver.closed). Placed under src/ (the uni project) since they have no external deps.
• npx vitest --run --project uni packages/telemetry/src — 16 pass.
• Full uni suite — 1033 pass / 1 skipped (no regressions).
• Existing int telemetry suites (metrics, propagation) — 30 pass.
• cd packages/telemetry && npx tsc --noEmit clean; oxlint packages/telemetry/src clean.

Checklist

• Changeset added (if package changes)
• README updated (if public API changed)