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