Setup monitoring for staging-ref

Part of immediate short term plan in gitlab-org&6401

As a first iteration, this issue will be to add basic health monitoring to tell us if gstg-10k is up and healthy.

https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/14054 will follow up with monitoring and alerting in line with our current production alerts https://gitlab.com/gitlab-com/runbooks

cc @amyphillips