Add support for aggregation of percentiles

Currently, aggregation across nodes is not supported for percentiles. We get percentiles per node, but not per bucket. Please add support for aggregation of percentiles.

 

Example:

http://172.23.97.21:8091/pools/default/stats/range/kv_bg_load_seconds_bucket/rate/p99?bucket=bucket-1&timeWindow=1m

{"data":[{"metric":

{"nodes":["172.23.97.21:8091"],"bucket":"bucket-1","instance":"kv","name":"kv_bg_load_seconds_bucket"}

,"values":[[1617829059,"0.0019306672701555937"],[1617829069,"0.0019026713342898119"],[1617829079,"0.0018935082728592195"],[1617829089,"0.0019081941394025602"],[1617829099,"0.0019040211798561097"],[1617829109,"0.0018861424163027643"],[1617829119,"0.0018851555555555576"]]},{"metric":

{"nodes":["172.23.97.22:8091"],"bucket":"bucket-1","instance":"kv","name":"kv_bg_load_seconds_bucket"}

,"values":[[1617829059,"0.0019301154417413566"],[1617829069,"0.0019390788067444919"],[1617829079,"0.0019379931428571417"],[1617829089,"0.001938005676037479"],[1617829099,"0.0019428804336043403"],[1617829109,"0.0019210472727272717"],[1617829119,"0.0019098450969529095"]]},{"metric":

{"nodes":["172.23.97.23:8091"],"bucket":"bucket-1","instance":"kv","name":"kv_bg_load_seconds_bucket"}

,"values":[[1617829059,"0.0019302261994609171"],[1617829069,"0.0019329838674033153"],[1617829079,"0.0019389013333333332"],[1617829089,"0.0019315806128133686"],[1617829099,"0.0019421343732590529"],[1617829109,"0.0019374270385126176"],[1617829119,"0.0019414934704370173"]]}],"errors":[],"startTimestamp":1617829059,"endTimestamp":1617829119}