docs: def get_field_description(f):
      """Get the type description of a GRPC Message field."""
      type_name = get_field_type(f)
      if type_name == 'MESSAGE' and \
              {sf.name for sf in f.message_type.fields} == {'key', 'value'}:
          return 'map<string, string>'
      elif type_name == 'MESSAGE':
          return f.message_type.full_name
      elif type_name == 'ENUM':
          return f.enum_type.full_name
      else:
          return type_name.lower()