Expose new registry Google CDN redirect metrics

This MR exposes the new application metrics exported by the Container Registry whenever the Google Cloud CDN feature is enabled:

https://dashboards.gitlab.net/dashboard/snapshot/9PvQr4G82hBDHQRvLO6JE4ogPm6VmWHI?orgId=1&var-PROMETHEUS_DS=Global&var-environment=gstg&var-stage=main&var-namespace=gitlab&var-Deployment=gitlab-registry&var-cluster=All&from=1642174798856&to=1642175698856

The screenshot above was taken during tests in Staging with the <code data-sourcepos="7:67-7:97">container_registry_cdn_redirect</code> feature flag set to 50%. The bulk of requests originated on my local machine, with some from within GCP (bypassed to GCS).

These metrics were introduced in gitlab-org/container-registry!830 (merged).

Related to https://gitlab.com/groups/gitlab-org/-/epics/7212 and https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/14934.