ds_kll_rank() receives two parameters: a STRING that represents a serialized DataSketches KLL sketch and a float to provide a probing value in the sketch.
Returns a DOUBLE that is the rank of the given probing value in the range of [0,1]. E.g. a return value of 0.2 means that the probing value given as parameter is greater than the 20% of all the values in the sketch. Note, this is an approximate calculation.