/// Returns the approximated cardinality of the union of the HyperLogLogs passed, by internally merging the HyperLogLogs stored at the provided keys into a temporary hyperLogLog, or 0 if the variable does not exist.
/// </summary>
/// <returns>The approximated number of unique elements observed via HyperLogLogAdd.</returns>
/// Merge multiple HyperLogLog values into an unique value that will approximate the cardinality of the union of the observed Sets of the source HyperLogLog structures.
/// Merge multiple HyperLogLog values into an unique value that will approximate the cardinality of the union of the observed Sets of the source HyperLogLog structures.
/// Returns the approximated cardinality of the union of the HyperLogLogs passed, by internally merging the HyperLogLogs stored at the provided keys into a temporary hyperLogLog, or 0 if the variable does not exist.
/// </summary>
/// <returns>The approximated number of unique elements observed via HyperLogLogAdd.</returns>
/// Merge multiple HyperLogLog values into an unique value that will approximate the cardinality of the union of the observed Sets of the source HyperLogLog structures.
/// Merge multiple HyperLogLog values into an unique value that will approximate the cardinality of the union of the observed Sets of the source HyperLogLog structures.