Use MergeRequest#merge_base_pipeline for comparing Metrics reports [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This MR adds feature-flagged usage of MergeRequest#merge_base_pipeline for comparing generic metrics reports. We decided to do this after the success of discerning introduced CodeQuality violations this way, and believe the designs hould be more reliable for Generic Metrics as well.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. There is no changelog entry while the feature is off by default.
• 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.

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.