feat: DocumentWriter returns number of documents written (#5939)

* Make DocumentWriter return the number of documents it wrote

* Fixed return type

haystack/preview/components/writers/document_writer.py

@@ -44,18 +44,19 @@ def from_dict(cls, data: Dict[str, Any]) -> "DocumentWriter":
         data["init_parameters"]["policy"] = DuplicatePolicy[data["init_parameters"]["policy"]]
         return default_from_dict(cls, data)
 
+    @component.output_types(documents_written=int)
     def run(self, documents: List[Document], policy: Optional[DuplicatePolicy] = None):
         """
         Run DocumentWriter on the given input data.
 
         :param documents: A list of documents to write to the store.
         :param policy: The policy to use when encountering duplicate documents.
-        :return: None
+        :return: Number of documents written
 
         :raises ValueError: If the specified document store is not found.
         """
         if policy is None:
             policy = self.policy
 
         self.document_store.write_documents(documents=documents, policy=policy)
-        return {}
+        return {"documents_written": len(documents)}

releasenotes/notes/add-document-writer-number-of-documents-written-2c57f3a5d6ae2131.yaml

@@ -0,0 +1,4 @@
+---
+enhancements:
+  - |
+    Document writer returns the number of documents written.