feat: add metrics for number of total and applied migrations

mentioned in issue #1684 (closed)

mentioned in merge request gitlab-org/charts/gitlab!4530 (merged)

mentioned in merge request gitlab-org/build/CNG!2650 (merged)

mentioned in merge request omnibus-gitlab!8746 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!4810 (merged)

mentioned in merge request omnibus-gitlab!8782 (closed)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!4811 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!4812 (merged)