Preemption performance improvement and new metrics release notes entry
Preemption performance improvement and new metrics release notes entry

Reviewed at https://reviews.apache.org/r/56048/

thinker0/aurora,thinker0/aurora,apache/aurora,apache/aurora,apache/aurora,rdelval/aurora,thinker0/aurora,crashlytics/aurora,crashlytics/aurora,medallia/aurora,medallia/aurora,medallia/aurora,apache/aurora,rdelval/aurora,rdelval/aurora,medallia/aurora,rdelval/aurora,thinker0/aurora,crashlytics/aurora,apache/aurora,crashlytics/aurora,apache/aurora,rdelval/aurora,crashlytics/aurora,thinker0/aurora,medallia/aurora,rdelval/aurora,medallia/aurora,crashlytics/aurora,thinker0/aurora