Alerting when there are no autometrics-related metrics

[emschwartz]

We ran into an issue where one version of our API stopped producing autometrics-related metrics, probably because of some slight configuration change.

Should we add an alerting rule to trigger an alert if there are no autometrics-related metrics in the Prometheus instance? Are there cases where this would fire incorrectly and be annoying?

I think we can use the absent_over_time function for this purpose.

What do others think?