How do you monitor a MongoDB production cluster and what metrics matter?

Critical metrics and tooling: 1) MongoDB Atlas: built-in monitoring with alerts. 2) Self-hosted: MongoDB Exporter → Prometheus → Grafana. 3) Key metrics: Operation latency (op_counters.command, query, insert): alert if P99 > 100ms. Replication lag: alert if > 10s. Connection count: alert if > 80% of maxIncomingConnections. Oplog window: must be large enough for maintenance. Cache hit ratio: wiredT