Resolve "Background Migrations Visibility Improvement for GitLab engineers - Iteration 1"

What does this MR do and why?

In this MR, we are adding a new method to calculate the progress of the batched background migration. In the future, Gitlab engineers will be able to get this value via Chatops.

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #346357 (closed)