code: public TimeSeriesMetricDeltaSet mapOptional(Function<? super MetricValue, Optional<? extends MetricValue>> fn) {
          return values_
                  .map(
                          fn,
                          (x) -> x.entrySet().stream()
                                  .map((entry) -> apply_fn_optional_(entry, fn))
                  )
                  .mapCombine(
                          opt_scalar -> opt_scalar.map(TimeSeriesMetricDeltaSet::new).orElseGet(() -> new TimeSeriesMetricDeltaSet(MetricValue.EMPTY)),
                          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.