Monitor consul members
We've had instances of postgres secondaries missing from consul in https://gitlab.com/gitlab-com/infrastructure/issues/4415 and https://gitlab.com/gitlab-com/infrastructure/issues/4330.
It would be useful to have prometheus check and alert on e.g. the member count for "postgres", so we can detect that situation automatically.
There is a consul exporter available: https://github.com/prometheus/consul_exporter