Required Stops: Research and set a metric based threshold for background migrations

Since we started collecting timing information in gitlab-org/database-team/gitlab-com-database-testing#53 (closed), we should consider if we can use that formula to set guidelines on when a background migration can be finalized and if it might introduce a required stop.

Deliverable

• Analyize background migration times vs their expected times from teh formula in gitlab-org/database-team/gitlab-com-database-testing#53 (closed).
• Based on the analysis update our timing guidelines here - https://docs.gitlab.com/ee/development/migration_style_guide.html#how-long-a-migration-should-take
• Add the information to gitlab-org/database-team/gitlab-com-database-testing#110