docs: fix annoyance in Swagger GUI generated example

Signed-off-by: Matteo Mortari <matteo.mortari@gmail.com>

Author: tarilabs

api/openapi/model-registry.yaml

@@ -1360,6 +1360,9 @@ components:
           MetadataStringValue: "#/components/schemas/MetadataStringValue"
           MetadataStructValue: "#/components/schemas/MetadataStructValue"
       description: A value in properties.
+      example:
+        string_value: my_value
+        metadataType: MetadataStringValue
     MetadataIntValue:
       description: An integer (int64) property value.
       type: object