code: func (f *Formatter) FormatItem(c, fd string) (string, error) {
  	archetype := proto.MessageType(fd)
  	if archetype == nil {
  		return "", errors.Reason("unknown proto.Message type %q in formatter data", fd).Err()
  	}
  	msg := archetypeMessage(archetype)
  
  	// Convert from config to protobuf.
  	if err := luciProto.UnmarshalTextML(c, msg); err != nil {
  		return "", errors.Annotate(err, "failed to unmarshal text protobuf content").Err()
  	}
  
  	// Binary format.
  	bc, err := makeBinaryContent(msg)
  	if err != nil {
  		return "", errors.Annotate(err, "").Err()
  	}
  	return bc, nil
  }
docs: // FormatItem implements cfgclient.Formatter.