Adds three new Prometheus metrics to track backup repository
maintenance job execution:
- velero_maintenance_job_success_total: Counter for successful jobs
- velero_maintenance_job_failure_total: Counter for failed jobs
- velero_maintenance_job_duration_seconds: Histogram for job duration
Metrics use repository_name label to identify specific BackupRepositories.
Duration is recorded for both successful and failed jobs (when job runs),
but not when job fails to start.
Includes comprehensive unit and integration tests.
Fixes#9225
Signed-off-by: Shubham Pampattiwar <spampatt@redhat.com>