feat: retrieve external_documents add doc_metadata (#22496)

Co-authored-by: profilechenyang <profilechenyang@didiglobal.com>
Co-authored-by: Byron.wang <byron@dify.ai>

api/core/rag/retrieval/dataset_retrieval.py

@@ -537,6 +537,7 @@ class DatasetRetrieval:
                 retriever_from=invoke_from.to_source(),
                 score=item.metadata.get("score"),
                 content=item.page_content,
+                doc_metadata=item.metadata,
             )
             retrieval_resource_list.append(source)
         # deal with dify documents