[DOCS] Add ML metric functions (elastic/x-pack-elasticsearch#1349)
[DOCS] Add ML metric functions (elastic/x-pack-elasticsearch#1349)

* [DOCS] Add ML metric functions

* [DOCS] Remove beta note from ML metrics function

Original commit: elastic/x-pack-elasticsearch@0bd2e359ac0a09c256b334099a63e1b7ec44474f

gfyoung/elasticsearch,nknize/elasticsearch,nknize/elasticsearch,GlenRSmith/elasticsearch,strapdata/elassandra,robin13/elasticsearch,uschindler/elasticsearch,gingerwizard/elasticsearch,strapdata/elassandra,uschindler/elasticsearch,vroyer/elassandra,uschindler/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,gfyoung/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,robin13/elasticsearch,nknize/elasticsearch,GlenRSmith/elasticsearch,strapdata/elassandra,robin13/elasticsearch,robin13/elasticsearch,nknize/elasticsearch,uschindler/elasticsearch,coding0011/elasticsearch,HonzaKral/elasticsearch,HonzaKral/elasticsearch,coding0011/elasticsearch,vroyer/elassandra,GlenRSmith/elasticsearch,GlenRSmith/elasticsearch,GlenRSmith/elasticsearch,strapdata/elassandra,robin13/elasticsearch,gingerwizard/elasticsearch,gingerwizard/elasticsearch,HonzaKral/elasticsearch,coding0011/elasticsearch,scorpionvicky/elasticsearch,vroyer/elassandra,nknize/elasticsearch,scorpionvicky/elasticsearch,coding0011/elasticsearch,gfyoung/elasticsearch,gingerwizard/elasticsearch,coding0011/elasticsearch,scorpionvicky/elasticsearch,strapdata/elassandra,HonzaKral/elasticsearch,gfyoung/elasticsearch,scorpionvicky/elasticsearch,gfyoung/elasticsearch