rewrite bacula alerts (team#41633)

This stops using the freshness metric from the exporer for incremental backups as well because it makes the expression more confusing, and anyways we define our threshold in the alerts, not the exporters.

That kind of thinking is what we were doing in Nagios before, now threshold are not in the exporters, period.

We also reverse those expressions: instead of calculating time() - DIFF and comparing against the metric, we calculate time() - metric to get the actual age of the backup, and compare that against the threshold. Then we can use that value in the alert as well.