function cumulativeTransformOnlyMetrics (metricNames) {
  return cumulativeTransformSelective(mi => metricNames.includes(mi.metric))
}
Convert a nominal value to a interval value. Performs a derivative over time.

ie: convert a series that increments forever into an average over the last time period

@param {metricNames} list of metric names to apply transform to
@returns {function} a transform function