code: func (c *Config) AddFlagsPrefix(fs *flag.FlagSet, prefix string) {
  	fs.Var(&c.Level, prefix+"log-level",
  		"The logging level. Valid options are: debug, info, warning, error.")
  }
docs: // AddFlagsPrefix adds common flags to a supplied FlagSet with a prefix.
  //
  // A string prefix must be supplied which will be prepended to
  // each added flag verbatim.