code: func (d *Discovery) readFile(filename string) ([]*targetgroup.Group, error) {
  	fd, err := os.Open(filename)
  	if err != nil {
  		return nil, err
  	}
  	defer fd.Close()
  
  	content, err := ioutil.ReadAll(fd)
  	if err != nil {
  		return nil, err
  	}
  
  	info, err := fd.Stat()
  	if err != nil {
  		return nil, err
  	}
  
  	var targetGroups []*targetgroup.Group
  
  	switch ext := filepath.Ext(filename); strings.ToLower(ext) {
  	case ".json":
  		if err := json.Unmarshal(content, &targetGroups); err != nil {
  			return nil, err
  		}
  	case ".yml", ".yaml":
  		if err := yaml.UnmarshalStrict(content, &targetGroups); err != nil {
  			return nil, err
  		}
  	default:
  		panic(errors.Errorf("discovery.File.readFile: unhandled file extension %q", ext))
  	}
  
  	for i, tg := range targetGroups {
  		if tg == nil {
  			err = errors.New("nil target group item found")
  			return nil, err
  		}
  
  		tg.Source = fileSource(filename, i)
  		if tg.Labels == nil {
  			tg.Labels = model.LabelSet{}
  		}
  		tg.Labels[fileSDFilepathLabel] = model.LabelValue(filename)
  	}
  
  	d.writeTimestamp(filename, float64(info.ModTime().Unix()))
  
  	return targetGroups, nil
  }
docs: // readFile reads a JSON or YAML list of targets groups from the file, depending on its
  // file extension. It returns full configuration target groups.