velero: Improve backup alerts

The velero_backup_*_total metrics are counters and will only increase. It's better to calculate the rate of failed backups against the total amount of backups made. See also https://github.com/vmware-tanzu/velero/issues/2725#issuecomment-661577500.

For easier review I pushed the linting commit first, and a second actual code change commit.

Hi btw 😃 !