Merged PR 2232: KPI refactor - 1st step, bookings

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

models/intermediate/cross/schema.yml

@@ -146,6 +146,24 @@ models:
           - not_null
           - unique
 
+  - name: int_mtd_vs_previous_year_metrics
+    description: |
+      This model is used for global KPIs.
+
+      It aggregates all the mtd models with the different metrics per source
+      and computes any necessary weighted metric across different sources.
+      Finally, it displays any metric on the current date, the previous year
+      date and it computes the relative increment by using the macro:
+      - calculate_safe_relative_increment
+
+    columns:
+      - name: date
+        data_type: date
+        description: The date for the month-to-date metrics.
+        tests:
+          - not_null
+          - unique
+
   - name: int_monthly_guest_revenue_history_by_deal
     description: |
       This model contains the historic information regarding the guest revenue for each deal id.