Support reducers in RedisTimeSeries 1.8 GA

[atakavci]

Simply adding the list of reducers coming with RedisTimeSeries 1.8;

avg Arithmetic mean of all non-NaN values
range Difference between maximum non-NaN value and minimum non-NaN value
count Number of non-NaN values
std.p Population standard deviation of all non-NaN values
std.s Sample standard deviation of all non-NaN values
var.p Population variance of all non-NaN values
var.s Sample variance of all non-NaN values

for more info https://redis.io/docs/latest/commands/ts.mrange/