code: func (ld *logData) viewerURL() string {
  	for _, s := range ld.logStreams {
  		desc, err := s.DescriptorValue()
  		if err != nil {
  			continue
  		}
  		if u, ok := desc.Tags["logdog.viewer_url"]; ok {
  			return u
  		}
  	}
  	return ""
  }
docs: // viewerURL is a convenience function to extract the logdog.viewer_url tag
  // out of a logStream, if available.
  // If given multiple streams, it will return the first one found with the viewer_url tag.