code: public Stream<Entry<Tags, MetricValue>> streamAsMap(Set<Tags> dfl) {
          return values_
                  .mapCombine(v -> dfl.stream().map(tag -> SimpleMapEntry.create(tag, v)), map -> map.entrySet().stream());
      }
docs: Retrieve the values in this set, mapped by their tags.
  @param dfl The tags to apply, if this collection holds a scalar.
  In this case, a metric value will be emitted for each tag.
  @return A stream of tagged metric-value entries.