record atom pull time

output looks like this:
********Pulled Atom stats***********
Atom 10003->(total pull)2, (pull from cache)0, (min pull interval)0,
(average pull time nanos)528047, (max pull time nanos)568386, (average
pull delay nanos)4222422, (max pull delay nanos)4498125
Atom 10019->(total pull)2, (pull from cache)0, (min pull interval)0,
(average pull time nanos)4455885, (max pull time nanos)6245938, (average
pull delay nanos)12153985, (max pull delay nanos)13518387
Atom 10020->(total pull)2, (pull from cache)0, (min pull interval)0,
(average pull time nanos)3092005, (max pull time nanos)3223594, (average
pull delay nanos)18301668, (max pull delay nanos)19805783
Atom 10021->(total pull)3, (pull from cache)0, (min pull interval)0,
(average pull time nanos)3151944, (max pull time nanos)4931719, (average
pull delay nanos)17319897, (max pull delay nanos)24455211

Bug: 118757386
Test: unit test
Change-Id: Idbe0e4cc822d75813d2e4511be5e7593fff370d4