Assign blessed_key to metadata if blessed_key is defined (faust-strea…

…ming#422)

* Assign blessed_key to metadata if blessed_key is defined

* just assign _blessed_key from hereon

faust/models/record.py

@@ -592,7 +592,7 @@ def to_representation(self) -> Mapping[str, Any]:
         payload = self.asdict()
         options = self._options
         if options.include_metadata:
-            payload["__faust"] = {"ns": options.namespace}
+            payload[self._blessed_key] = {"ns": options.namespace}
         return payload
 
     def asdict(self) -> Dict[str, Any]:  # pragma: no cover