Instrument sub-transactions created by ActiveRecord

What does this MR do?

We've experience performance degradations related to SubtransControlLocks blocking CI/CD builds queuing queries. We want to understand where do the subtransactions come from. See gitlab-com/gl-infra/production#5152 (comment 629653355) for more details.

Conformity

• I have added information for database reviewers in the MR description.
• I have self-reviewed this MR per code review guidelines.
• This change is backwards compatible across updates.