feat(gc): add event label to online GC run metrics

mentioned in commit fda98a18

mentioned in issue #536 (closed)

mentioned in merge request gitlab-org/build/CNG!844 (merged)

mentioned in merge request gitlab-org/charts/gitlab!2312 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1414 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1415 (merged)

mentioned in merge request omnibus-gitlab!5827 (merged)

mentioned in merge request gitlab-development-kit!2385 (merged)