code: public static int getSparseStorageSize(int nonEmptyBuckets)
    {
      return COMMON_FIELDS_SIZE +
             Integer.BYTES +
             (Integer.BYTES + Long.BYTES) * nonEmptyBuckets;
    }
docs: Compute the size in bytes of a sparse-encoding serialized histogram, without the serialization header
  
  @param nonEmptyBuckets number of non-empty buckets
  @return sparse serialized size in bytes