monitoring: wrap alert_count records in min/max

Created by: bobheadxi

Wrap all alert_count records in a min/max operators, and removes "support" for per-instance since this eliminates per-container / per-replica / per-pod labels from the final alert_count rule calculation. This closes #11571, resolving a bug where per-instance support in provisioning alerts were causing them to not apply. See https://github.com/sourcegraph/sourcegraph/issues/11571#issuecomment-654571953

Adjust wording and descriptions on the provisioning alerts.

I tried this out with new gitserver rules via the steps I described here on k8s and provisioning alerts seem to be green now