Use upsert for MR metrics

What does this MR do?

This MR changes the ensure_metrics method for MergeRequest to use an UPSERT query. The main goal is to avoid the subtransaction described by #338960 (closed)..

The upsert query is behind a FF: use_upsert_query_for_mr_metrics

The enesure_metrics is thoroughly tested at: https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/models/merge_request_spec.rb#L272

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #338960 (closed)