Add feature flag mechanism to aggregated metrics

What does this MR do?

This is follow up MR do discussion made at !46953 (comment 443105042)

It implements ability to hide each metrics aggregation behind dedicated development feature flag, this will allow for more control over aggregated metrics rollout procedure

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry All aggregated metrics are hidden behind feature flag so far #267550 (closed)
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content