feat: add histogram metric to monitor the number of imported tags

mentioned in commit 517ff5c7

mentioned in issue #646 (closed)

mentioned in merge request gitlab-org/build/CNG!973 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1707 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1708 (merged)

mentioned in merge request gitlab-org/charts/gitlab!2510 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1715 (merged)

mentioned in merge request omnibus-gitlab!6064 (merged)

mentioned in merge request gitlab-development-kit!2503 (merged)