Make metadata required in Vertex AI Index resource (#14382)

Author: jaylonmcshan19-x

mmv1/products/vertexai/Index.yaml

@@ -84,7 +84,10 @@ properties:
   # https://cloud.google.com/vertex-ai/docs/matching-engine/configuring-indexes
   - name: 'metadata'
     type: NestedObject
-    description: An additional information about the Index
+    description: |-
+      Additional information about the Index.
+      Although this field is not marked as required in the API specification, it is currently required when creating an Index and must be provided.
+      Attempts to create an Index without this field will result in an API error.
     properties:
       - name: 'contentsDeltaUri'
         type: String