Add Prometheus metrics and Grafana dashboards

[piwi3910]

Description

Add comprehensive observability infrastructure for NovaStor:

• Prometheus metrics registration for all components (agent, metadata, controller, CSI, S3, filer)
• Convenience collector for agent metrics
• Grafana cluster overview dashboard
• Prometheus alerting rules

Acceptance Criteria

• internal/metrics/metrics.go with all metric registrations
• internal/metrics/collector.go with AgentCollector stub
• deploy/monitoring/grafana-dashboard-overview.json with cluster overview dashboard
• deploy/monitoring/prometheus-rules.yaml with alerting rules
• Code compiles successfully (go build ./internal/metrics/)

Labels

enhancement, observability

[piwi3910]

This issue has been resolved. The following files are now in place:

• internal/metrics/metrics.go - Prometheus metric registrations for all components
• internal/metrics/collector.go - AgentCollector implementation
• deploy/monitoring/grafana-dashboard-overview.json - Cluster overview dashboard
• deploy/monitoring/grafana-dashboard-agent.json - Agent-specific dashboard
• deploy/monitoring/prometheus-rules.yaml - Alerting rules

All acceptance criteria have been met.