Create index asynchronously for system_note_metadata (!141330) · Merge requests · GitLab.org / GitLab

What does this MR do and why?

system_note_metadata table's primary key column id needs to be converted to use bigint type.

id_convert_to_bigint is the new column that's been backfilled and uses bigint. We need to create a new index on the column. The index will be used to support the new primary key (this step is a part of https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#swap-the-columns-release-n--1). The new index take a long time to create and will need to be prepared asynchronously for .com in this MR.