etcdserver/stats: fix stats data race.
etcdserver/stats: fix stats data race.

li-ang/etcd,bdarnell/etcd,rnaveiras/etcd,ajityagaty/etcd,cockroachdb/etcd,gyuho/etcd,joshix/etcd,rnaveiras/etcd,xiang90/etcd,mqliang/etcd,etcd-io/etcd,bdarnell/etcd,mqliang/etcd,etcd-io/etcd,ajityagaty/etcd,cockroachdb/etcd,xiang90/etcd,dannysauer/etcd,mqliang/etcd,sinsharat/etcd,joshix/etcd,sebrandon1/etcd,ajityagaty/etcd,sebrandon1/etcd,mkumatag/etcd,lorneli/etcd,dannysauer/etcd,etcd-io/etcd,fanminshi/etcd,coreos/etcd,lorneli/etcd,mkumatag/etcd,glevand/coreos--etcd,rnaveiras/etcd,glevand/coreos--etcd,fanminshi/etcd,mkumatag/etcd,sinsharat/etcd,vimalk78/etcd,li-ang/etcd,coreos/etcd,vimalk78/etcd,gyuho/etcd