WIP: Send backup metrics to Prometheus in gitlab:backup:create

What does this MR do?

Adds Prometheus metrics to the gitlab:backup:create rake task for backup size, start time, end time, and duration.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Backups are important and this allows setting up an alert if something went wrong (i.e. no new successful backup for a certain amount of time)

Having a history of the size of backups allows to better plan for future storage requirements.

Screenshots (if relevant)

Not a user-visible feature.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes #34362 (moved)