Add Prometheus metrics for all five services #37
Description
Summary
Instrument all five services with Prometheus metrics: ingestion rate, correlation latency, delivery success, RPC health, finality queue depth.
Changes
- prom-client integration per service
- Stripe events ingested/sec (by event type)
- Chain transactions detected/sec (by chain)
- Correlation success/failure rate and latency histogram
- Webhook delivery success rate and p99 latency
- RPC health status gauge per chain per provider
- /metrics endpoint per service
Tests
- Metrics endpoint returns valid Prometheus format
- Counters increment on expected events
Acceptance Criteria
All services expose comprehensive Prometheus metrics.