feat: improve metadata handling in connector service (#11421)

### What problem does this PR solve?

- Update sync data source to handle metadata properly

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

---------

Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

common/data_source/discord_connector.py

@@ -65,6 +65,7 @@ def _convert_message_to_document(
         blob=message.content.encode("utf-8"),
         extension=".txt",
         size_bytes=len(message.content.encode("utf-8")),
+        metadata=metadata if metadata else None,
     )