in api-generator/main.go [694:703]
func buildExtension(scope *string, filename *string, syntax ptype.Syntax, extension *descriptor.FieldDescriptorProto, output []*ptype.Field) ([]*ptype.Field, error) {
field, err := buildMessageField(extension)
if err != nil {
return nil, err
}
output = append(output, field)
return output, nil
}