code: public TimeSeriesMetricDeltaSet map(Function<? super MetricValue, ? extends MetricValue> fn) {
          return values_
                  .map(
                          fn,
                          (x) -> x.entrySet().stream().map((entry) -> apply_fn_(entry, fn))
                  )
                  .mapCombine(TimeSeriesMetricDeltaSet::new, TimeSeriesMetricDeltaSet::new);
      }
docs: Apply a single-argument function to the set.
  @param fn A function that takes a TimeSeriesMetricDelta and returns a TimeSeriesMetricDelta.
  @return The mapped TimeSeriesMetricDelta from this set.