Add Prometheus exporter for DBLab metrics

Summary

Add a Prometheus exporter to expose DBLab Engine metrics for monitoring.

Metrics to expose

• Free disk space
• Number of clones, snapshots, branches
• Max age of clones
• Max age of snapshots
• Max diff for snapshots (physical mode)
• Non-busy disk slots (datasets) in logical mode
• CPU load and RAM usage per clone
• Disk usage (extra) per snapshot
• Disk usage (extra) per clone

Benefits

• Enable integration with Prometheus-based monitoring systems
• Support alerting on key metrics (disk space, clone count, data freshness)
• Provide visibility into resource usage per clone

---

Acceptance Criteria

• A Prometheus exporter endpoint (e.g., /metrics) is exposed by the DBLab Engine serving metrics in Prometheus text format
• All specified metrics are exported: free disk space, clone count, snapshot count, branch count, max clone age, max snapshot age, max snapshot diff, non-busy disk slots, CPU/RAM per clone, disk usage per snapshot and per clone
• Metrics are labeled with appropriate dimensions (clone ID, snapshot ID, etc.) for filtering and aggregation
• The exporter works alongside the existing DBLab API without port conflicts
• Prometheus can scrape the endpoint and visualize metrics in Grafana

Definition of Done

• Prometheus exporter implemented and merged
• All listed metrics emit correct values verified against DBLab API responses
• Grafana dashboard template created for DBLab metrics visualization
• Documentation updated with Prometheus scrape configuration examples
• Tested with a real Prometheus instance scraping metrics over a sustained period