Merged PR 2317: Exposing first_day_month for business kpis

This PR exposes the field first_day_month in the business KPIs, for the Global view. It's just to be able to properly display graphs, since it's quite confusing doing it so by last_day_month (see screenshot)![image.png](https://guardhog.visualstudio.com/4148d95f-4b6d-4205-bcff-e9c8e0d2ca65/_apis/git/repositories/54ac356f-aad7-46d2-b62c-e8c5b3bb8ebf/pullRequests/2317/attachments/image.png)

Related work items: #18580

models/reporting/general/mtd_aggregated_metrics.sql

@@ -9,6 +9,7 @@ select
     day as day,
     is_end_of_month as is_end_of_month,
     is_current_month as is_current_month,
+    first_day_month as first_day_month,
     date as date,
     previous_year_date as previous_year_date,
     order_by as order_by,

models/reporting/general/schema.yaml

@@ -345,6 +345,14 @@ models:
         tests:
           - not_null
 
+      - name: first_day_month
+        data_type: date
+        description: |
+          first day of the month correspoding to the date field. 
+          It comes from int_dates_mtd logic.
+        tests:
+          - not_null
+
       - name: date
         data_type: date
         description: |