Use default_enabled: :yaml in aggregated metrics

What does this MR do?

Initial implementation of Aggregated Metrics feature was implemented before default_enabled: :yaml option was added for feature flags, to make usage of development feature flags for new aggregates more flexible fixed default_enabled: false should be changed to default_enabled: :yaml

Does this MR meet the acceptance criteria?

Conformity

• [ ] Changelog entry tooling change
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides