[FEATURE] - Observability: Prometheus metrics and audit logging

[patrick-hermann-sva]

Summary

Add Prometheus metrics and structured audit logging to provide observability into clusterbook's operations, pool utilization, and allocation activity.

Benefits

• Alerting on pool exhaustion before it becomes a problem
• Visibility into allocation rates and patterns
• Audit trail for compliance and troubleshooting
• Integration with existing monitoring stacks (Prometheus/Grafana)

Suggested Metrics

• clusterbook_pool_total{network} — total IPs per pool
• clusterbook_pool_assigned{network} — assigned IPs per pool
• clusterbook_pool_available{network} — available IPs per pool
• clusterbook_allocations_total — counter of allocation requests
• clusterbook_allocation_errors_total — counter of failed allocations

Suggested Approach

• Expose /metrics endpoint using prometheus/client_golang
• Add structured JSON logging for all allocation/deallocation events
• Include Grafana dashboard JSON in the Helm chart
• Add ServiceMonitor CR template for Prometheus Operator

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