ENGCOM-2707: [Backport] Fix #4803: Incorrect return value from Product Attribute Repository #15688

 - Merge Pull Request #15688 from cream-julian/magento2:2.1-develop-issue-4803
 - Merged commits:
   1. d409248
   2. 90f37dc
   3. b064cd7

Author: magento-engcom-team

app/code/Magento/Eav/Api/Data/AttributeInterface.php

@@ -10,7 +10,9 @@
  * Interface AttributeInterface
  * @api
  */
-interface AttributeInterface extends \Magento\Framework\Api\CustomAttributesDataInterface
+interface AttributeInterface extends
+    \Magento\Framework\Api\CustomAttributesDataInterface,
+    \Magento\Framework\Api\MetadataObjectInterface
 {
     const ATTRIBUTE_ID = 'attribute_id';