make use of our common monitoring module

Rather than ship our own monitoring module, make use of the gcp-site-monitoring module. This effectively makes this module require terraform 0.13 so a major version bump is required. Once this is merged, "release" by opening a v2 branch and having the previous master be v1 as outlined in the README.

This will add monitoring for the auto-generated Cloud Run domain as well as any domain mappings. Allow monitoring to be disabled if, for example, the service is configured to disallow direct invocations.

Tested by upgrading the tlscerts deployment to terraform 0.13 (https://gitlab.developers.cam.ac.uk/uis/devops/tls-certificates/google-cloud-deploy/-/merge_requests/16) and deploying development:

Closes #13 (closed)