Keyhole is a tool used to monitor MongoDB clusters by integrating with Grafana and analyzing Full Time Diagnostic data (FTDC) metrics. It helps identify performance bottlenecks in MongoDB clusters, including WiredTiger tickets, disk IOPS, CPU usage, replication lags, and slow operations from logs. Keyhole can be integrated with Grafana as a SimpleJson Datasource to display FTDC metrics visually, making it easier to diagnose cluster health issues. By analyzing FTDC data and mongo logs, Keyhole provides insights into potential problems such as running out of read or write tickets, disk performance issues, and slow database operations. The tool can also be used to verify new installations, collect MongoDB cluster information, and visualize resource usages in a snapshot.