code: def set_metadata(self, key: str, value: str):
          
          if not isinstance(key, str) or not isinstance(value, str):
              raise TypeError("'key' and 'value' of metadata MUST be strings")
          self.metadata[key] = value
docs: Add a new metadata to the message
  
          Args:
            key (str): name of the metadata
            value (str): value of the metadata