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.
The computed merged HyperLogLog is set to the destination variable, which is created if does not exist (defaulting to an empty HyperLogLog).
*Return value
Simple string reply: The command just returns OK.
*Examples
redis>
PFADD hll1 foo bar zap a
(integer) 1redis> PFADD hll2 a b c foo
(integer) 1redis> PFMERGE hll3 hll1 hll2
OKredis> PFCOUNT hll3
(integer) 6