code: func (logger *Logger) WithField(key string, value interface{}) logging.LogWithLevel {
  	return logger.withFields(logging.Fields{key: value}, 1)
  }
docs: // WithField creates an entry from the standard logger and adds a field to
  // it. If you want multiple fields, use `WithFields`.
  //
  // Note that it doesn't log until you call Debug, Print, Info, Warn, Fatal
  // or Panic on the LogMsg it returns.