The following existing index
'idx_issues_on_project_id_and_created_at_and_id_and_state_id'
has `state_id` as the last column.

Because GitLab's domain logic usually requires querying
issues in a particular state and the primary key comes before
state_id, the above existing index could prevent a range scan.

The newly added index modifies the order of the columns.
(project_id, state_id, created_at, id).

We will add the new index and collect data on the old and new
indices before disabling/eventually retiring one of them.

Changelog: added