Log CI artifact size when downloaded via UI

What does this MR do and why?

Previously we only logged the CI artifact size when the artifact was downloaded via an API endpoint. This commit adds a meta.artifact_size log entry for the ArtifactsController#download case as well.

How to set up and validate locally

1. Go to a job and download its artifact.
2. Check /var/log/gitlab/gitlab-rails/production_json.log for meta.artifact_size:

  "meta.user": "root",
  "meta.user_id": 1,
  "meta.project": "root/simple-ci",
  "meta.root_namespace": "root",
  "meta.client_id": "user/1",
  "meta.artifact_size": 209,
  "meta.artifact_used_cdn": true,

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.