Sentinel/Cluster failover event tracking with slowlog correlation

[KIvanow]

Description

Track Sentinel and cluster failover events and correlate them with slowlog/COMMANDLOG spikes for post-incident analysis.

Problem

During failovers, latency spikes and command failures are common but the cause isn't always obvious from metrics alone. Being able to see "a failover happened at 03:12, and here's the slowlog spike that preceded/followed it" would make post-incident analysis significantly easier.

Proposed Scope

• Detect failover events by monitoring INFO replication role changes and CLUSTER INFO state transitions
• Persist failover events with timestamps, old/new primary, and trigger reason where available
• Correlate failover timestamps with existing slowlog and anomaly detection data
• Surface in the UI timeline alongside existing anomaly events
• Add failover.started and failover.completed webhook event types

Prior Art / Context

Requested by community — correlating failovers with slowlog spikes is a common post-incident debugging need for teams running Sentinel or Cluster topologies.

Related

• Existing cluster topology visualization
• Existing per-slot heatmaps and migration tracking
• Anomaly detection correlator (could add a FAILOVER pattern)