Fix basic alerting so that it... alerts.

Since adding the alerting I find that it doesn't work any more. I'm not sure why since I did test at the time. We're now using a newer terraform provider - maybe that's it, or perhaps the underlying api in gcloud has changed. Anyway, I believe that this works - it's quite hard to test without actually using it. At the time of writing the dev shib-metadata project uses this. See e.g https://console.cloud.google.com/monitoring/uptime?project=shib-metadata-devel-d10d66c7

See also https://gitlab.developers.cam.ac.uk/uis/devops/raven/shibboleth-metadata-deployment/-/issues/8 https://gitlab.developers.cam.ac.uk/uis/devops/tls-certificates/google-cloud-deploy/-/issues/8