Add alerts to the reporting indexers

[paul-butcher]

Relates to #2520

A bug went unnoticed for some time because we did not have an alert to tell us either that the ECS Deployments were failing or that messages were expiring without being processed. I discovered the issue because something alphabetically adjacent did raise an alarm, and I happened to spot a surprisingly large number in a nearby row that wasn't getting any smaller.

We should send a Slack alert when this starts happening.