feat: add stable event handler event metrics

[adityathebe]

• add event handler metrics for event-level status and duration\n- instrument sync and async consumers to record success/failed counts\n- add named event handler registration APIs and use stable handler names in notification/playbook registrations

# HELP event_handler_events_total Total number of events processed by event handlers.
# TYPE event_handler_events_total counter
event_handler_events_total{event="config.changed",handler="notification.addNotificationEvent",status="success"} 287
event_handler_events_total{event="config.changed",handler="playbook.Handle",status="success"} 287
event_handler_events_total{event="config.healthy",handler="notification.addNotificationEvent",status="success"} 1
event_handler_events_total{event="config.healthy",handler="playbook.Handle",status="success"} 1
event_handler_events_total{event="config.unknown",handler="notification.addNotificationEvent",status="success"} 1
event_handler_events_total{event="config.unknown",handler="playbook.Handle",status="success"} 1
event_handler_events_total{event="config.updated",handler="notification.addNotificationEvent",status="success"} 55
event_handler_events_total{event="config.updated",handler="playbook.Handle",status="success"} 55

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