fix: increment and decrement inflight imports metric correctly

mentioned in commit 9772df75

mentioned in issue #627 (closed)

mentioned in merge request gitlab-org/build/CNG!938 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1612 (merged)

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

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1614 (merged)

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

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