[Tech Debt] Extend unit tests for metrics

[jbiers]

When implementing metrics for the Backup&Restore Operator a mix of integration and unit tests was used to make sure the metrics are working as expected. Integration tests were prioritized, while unit tests were used strictly when using e2e were not possible.

Ideally the project should have unit tests covering all metrics.

https://github.com/rancher/backup-restore-operator/blob/main/pkg/monitoring/metrics_test.go