@Override
    public long estimateMemoryUsageBytes() {
      return estimateSize(keyToValuesIndex) + estimateSize(values);
    }
A map from {@code long} to {@code long} where each putAll replaces previous values and results are returned in the
same order they were inserted.
planetiler-core/src/main/java/com/onthegomap/planetiler/collection/LongLongMultimap.java