code: function (startTime, endTime, callback) {
      var self = this;
  
      self._request({
        path: urlJoin(_urlPrefix, 'usage'),
        qs: {
          startTime: typeof startTime === 'Date' ? startTime.toISOString() : startTime,
          endTime: typeof endTime === 'Date' ? endTime.toISOString() : endTime
        }
      }, function (err, body) {
        return callback(err, body);
      });
    }
docs: client.getAccountUsage
  
  @description lists account level usage
  
  @param {Date|String}     startTime     the start time for the query
  @param {Date|String}     endTime       the end time for the query
  @param {function}        callback