First step on refactor of kpis:
- Remove relative incremental vs. previous year computation from the source model (`mtd_booking_metrics`, in this case)
- Aggregate the source mtd global metrics models into a single model: `int_mtd_vs_previous_year_metrics` (to enable multi-source weighted metric computation) and compute previous year value and relative increment. Now this logic is encapsulated into a macro `calculate_safe_relative_increment`, easing readability and providing a bit more robustness.
- End-to-end continuity to not break the existing dashboard display in `int_core__mtd_aggregated_metrics`
This is a substep of the global change. All info can be found in the documentation [here](https://www.notion.so/knowyourguest-superhog/Refactoring-Business-KPIs-5deb6aadddb34884ae90339402ac16e3)
Related work items: #18202
