feat: add metrics for max concurrent imports

mentioned in commit 2541e13b

mentioned in issue #619 (closed)

mentioned in merge request gitlab-org/build/CNG!924 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1575 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1602 (merged)

mentioned in merge request gitlab-org/charts/gitlab!2458 (merged)

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

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