Add SLO alerting for Artifacts and Registry CDN

John Jarvisrequested to merge
jarv/add-cdn-metrics into master

Previously, we didn't have any SLO alerting for the Registry and Artifact CDN. This adds them both, one under the Registry service and the other under the cloud-storage service (for lacking a better place).

For https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/16466

Edited by John Jarvis

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