feat: add Prometheus metrics for backup recovery window

[ermakov-oleg]

Summary

Port of upstream #459, #467

Problem: No observability into backup health — operators had no way to alert on stale backups or monitor recovery point objectives (RPO) without manually querying pgBackRest.

Fix: Implements the cnpg-i Metrics service, exposing two Prometheus gauges:

• cnpg_pgbackrest_first_recoverability_point — unix timestamp of the earliest restore point (first successful backup stop time)
• cnpg_pgbackrest_last_available_backup_timestamp — unix timestamp of the most recent completed backup (latest backup stop time)

These allow standard Prometheus alerts like "no backup in last 24h" or "RPO exceeds 1h".

Implementation:

• New MetricsServiceImplementation in internal/cnpgi/instance/metrics.go
• Registers TYPE_METRICS capability in plugin identity
• Collect() calls pgbackrest info to get the backup catalog, then delegates to getRecoveryWindow() which uses catalog.FirstRecoverabilityPoint() and catalog.GetLastSuccessfulBackupTime() — these methods filter out errored backups (Start=0 or Stop=0) and use Time.Stop for recoverability
• Returns 0 for both metrics if no backups exist or credentials fail (graceful degradation)

Unit tests in metrics_test.go cover: nil/empty catalog, single backup, multiple backups, errored backups filtering, all-errored catalog.

Related issues

• Is it possible to see the backup's data with the CloudNativePG prometheus dashboard? #32
• How to run pgbackrest info or an equivalent #19

[ermakov-oleg]

Hi @Agalin, just following up on this PR - would you have a chance to review it when you have time? The changes from all my PRs have been running in our production for a while now without issues, but I’m happy to adjust anything if needed.