code: def metadata(self):
          
          # Check exceptional case: return none if no metadata
          if not self._operation.HasField('metadata'):
              return None
  
          # Return expected metadata
          return _from_any(self._metadata_type, self._operation.metadata)
docs: Returns the value of Operation.metadata from the last call to
          OperationsClient.get_operation (or if only the initial API call has been
          made, the metadata from that first call).