Added timestamp to a/b test model

models/intermediate/core/schema.yml

@@ -4719,6 +4719,15 @@ models:
         data_tests:
           - not_null
 
+      - name: first_appearance_at_utc
+        data_type: date
+        description: |
+          Timestamp of the first Verification Request Feature Flag record created 
+          with the same verification request and A/B test name. Mostly for information 
+          purposes. Cannot be null.
+        data_tests:
+          - not_null
+
       - name: first_appearance_date_utc
         data_type: date
         description: |