feat(metrics): include monitoring for api nginx

Part of gitlab-com/gl-infra&340 (closed)

This follows on from !3540 (merged)

Required for gitlab-com/gl-infra/delivery#1692 (closed)

This introduces SLIs for the nginx ingress monitoring for the api service.

At present, nginx ingress controllers don't export a stage label, so these metrics don't have a stage label at present.

Once gitlab-com/gl-infra/delivery#1727 (closed) is delivered, these stage label should be reflected in these metrics