Model nginx ingress as part of the metrics-catalog, using our standard label taxonomy. Nginx ingress should be treated as an SLI with labels for `type`, `tier`, `stage`, `shard` and an SLO. Luckily nginx ingress appears to have some fairly good metrics, but we may need to adjust the labels that we're using. Implementation in https://gitlab.com/gitlab-com/runbooks/-/merge_requests/3544