Add database sourced aggregated metric [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This MR aims to wrap remaining pieces to complete implementation of &5227 (closed), in order to achieve this it introduces following changes:

1. It adds first database sourced aggregated metric definition - this was done, to provide real use case for validation all changes in production env, as well as to discover some minor problems, with underlying implementation of aggregated metrics for database sourced metrics feature.
2. It splits aggregated metrics definition for CE and EE, to avoid raising errors when metric in not present in EE

Relates to #300411 (closed) as it is impossible to verify if feature works correctly without any database sourced aggregate defined

Does this MR meet the acceptance criteria?

Conformity

• [ ] Changelog entry change behind feature flag
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content