Add metrics endpoint and monitoring for event processing

[robertocarlous]

Summary

Provide observability: expose Prometheus-style metrics for event processing, agent loop health, processing latency, error counts, and DLQ size.

What to do

• Add GET /metrics (or integrate prom-client) to expose counters/histograms:
• • events_processed_total
  • events_failed_total
  • event_processing_duration_seconds
  • dlq_size
  • event_cursor_lag
  • agent_loop_heartbeat_timestamp
• Instrument startEventListener(), event processing pipeline, DLQ retries, and agent loop with metrics.
• Add documentation on key metrics and recommended alerting thresholds.
• Optionally add sample Grafana dashboard panels in docs.

Acceptance criteria

• /metrics endpoint returns Prometheus-format metrics.
• Key metrics are collected and documented.
• Example alerts documented (e.g., dlq_size > 50, event_cursor_lag > X).

[Windowlight]

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[johnsmccain]

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[Blaqkenny]

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[dotmantissa]

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