Add temporary column `new_amount_used` for CI minutes tracking

What does this MR do and why?

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/364282

In this MR we want to increase the accuracy of ci_project_monthly_usages.amount_used from 2 decimal figures to 4 decimal figures.

In doing that we add a new_amount_used column and set a trigger to keep that in sync with amount_used. In a separate MR (for release N+1) we would remove the old column and rename the new one to amount_used:

transaction do
  remove_column :amount_used
  rename_column :new_amount_used, :amount_used
   
  # check if trigger is dropped automatically or drop the trigger
  # explicitly inside the transaction.
end

Database migration

$ rails db:migrate

ci: == 20221010184839 AddNewAmountUsedToCiProjectMonthlyUsages: migrating =========
ci: -- add_column(:ci_project_monthly_usages, :new_amount_used, :decimal, {:default=>0.0, :precision=>18, :scale=>4, :null=>false, :if_not_exists=>true})
ci:    -> 0.0077s
ci: == 20221010184839 AddNewAmountUsedToCiProjectMonthlyUsages: migrated (0.0167s) 

$ rails db:rollback:ci

ci: == 20221010184839 AddNewAmountUsedToCiProjectMonthlyUsages: reverting =========
ci: -- remove_column(:ci_project_monthly_usages, :new_amount_used)
ci:    -> 0.0014s
ci: == 20221010184839 AddNewAmountUsedToCiProjectMonthlyUsages: reverted (0.0065s) 

Data migration

• Count of records to update:

  • SELECT COUNT(*) FROM "ci_project_monthly_usages" WHERE (amount_used > 0)
  • https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/12884/commands/45252

• Find first ID for batching:

  • SELECT "ci_project_monthly_usages"."id" FROM "ci_project_monthly_usages" WHERE (amount_used > 0) ORDER BY "ci_project_monthly_usages"."id" ASC LIMIT 1
  • https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/12884/commands/45249

• Find first batch:

  • SELECT “ci_project_monthly_usages”.“id” FROM “ci_project_monthly_usages” WHERE (amount_used > 0) AND “ci_project_monthly_usages”.“id” >= 1 ORDER BY “ci_project_monthly_usages”.“id” ASC LIMIT 1 OFFSET 500
  • https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/12884/commands/45250

• Update statement:

  • `SELECT "ci_project_monthly_usages".* FROM "ci_project_monthly_usages" WHERE (amount_used > 0) AND "ci_project_monthly_usages"."id" >= 1 AND "ci_project_monthly_usages"."id" < 4
  • https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/12884/commands/45251

TODO

• Create same migration for ci_namespace_monthly_usages
• Create background/post-deployment migration to SET new_amount_used = amount_used for existing records (in batches)

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