Wire hub OTel metric recorders to Cloud Monitoring

[ptone]

Problem

Hub-side OTel recorders (dbmetrics, dispatchmetrics) are created with NewDisabled() at cmd/server_foreground.go:1174. The SetDBMetrics() and SetDispatchMetrics() methods on hub.Server exist but are never called from any entrypoint. All hub operational metrics — pg LISTEN/NOTIFY latency, notifications published/delivered/dropped, subscriber lag, dispatch lifecycle, pool stats — silently record to no-op sinks.

Solution

Create a GCP Cloud Monitoring MeterProvider during hub server startup and wire the existing recorders to it. This is Phase 1 of the metrics-delivery project.

Changes

• New: pkg/observability/hubmetrics/hubmetrics.go — creates a MeterProvider with a GCP metric exporter and periodic reader (60s)
• Modified: cmd/server_foreground.go — wire SetDBMetrics() and SetDispatchMetrics() after initHubServer(), pass real recorder to newEventPublisher()
• Metric groups (db-notify, db-pool, dispatch, hub-auth, hub-gcp) can be independently disabled via env vars
• Graceful degradation when GCPProjectID is empty

Verification

• Unit tests pass for all observability packages
• Build succeeds
• Live infrastructure verification pending (metrics should appear in Cloud Monitoring when deployed with a valid GCP project ID)