function combineValuesByTitle (fn) {
  return function _combineValuesByTitle (metricInstances) {
    return metricInstances.reduce(combineValuesByTitleReducer(fn), [])
  }
}
Combine metrics sharing the same 'title', applying some function to combine them, for
example a sum function.

@param {function} fn the combiner (eg a sum function) that will be used to reduce across values
@return {function} a transform function