Add alert when gcp-service-discovery is failing to update

[stephen-soltesz]

sum by(service) (up{job="aeflex-targets"}) == 0

[stephen-soltesz]

I think this was due to some occurrence when the service account credentials were incorrect and the old lists had been downloaded but no new ones were downloaded and there was no way to tell.

The query above is not actually helpful -- this will require a code change to the service discovery service with monitorable metrics.