Default enable use_upsert_query_for_mr_metrics FF

What does this MR do and why?

This MR enables the use_upsert_query_for_mr_metrics feature flag by default. The flag is already enabled globally on PRD since 14.3.

Rollout issue: #339677 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.