For the composer image download — it would be the same from the runners point of view. he needs a docker image. it would not matter if it takes it from the hub.docker.com or from my registry.
but one could argue it would make sense to let the runner itself have the composer image stored and cached. the container is 56MB so its quite fast to pull.
The metrics are via gitlab pages, and published on the gitlab host itself. the task runs parallel with the deployment. its always an option to to set