feat(notifications): add endpoint name label to Prometheus metrics

mentioned in commit 4b67b49c

mentioned in issue #987 (closed)

mentioned in merge request gitlab-org/build/CNG!1369 (merged)

mentioned in merge request gitlab-org/charts/gitlab!3096 (merged)

mentioned in merge request omnibus-gitlab!6816 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!2663 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!2664 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!2666 (merged)

mentioned in merge request gitlab-development-kit!3152 (merged)