code: func (fl *Flags) Register(f *flag.FlagSet, defaults auth.Options) {
  	fl.defaults = defaults
  	f.StringVar(&fl.serviceAccountJSON, "service-account-json", fl.defaults.ServiceAccountJSONPath, "Path to JSON file with service account credentials to use.")
  	if fl.RegisterScopesFlag {
  		defaultScopes := strings.Join(defaults.Scopes, " ")
  		if defaultScopes == "" {
  			defaultScopes = auth.OAuthScopeEmail
  		}
  		f.StringVar(&fl.scopes, "scopes", defaultScopes, "space-separated OAuth 2.0 scopes")
  	}
  }
docs: // Register adds auth related flags to a FlagSet.