etcdserver: unset old cluster version in metrics
etcdserver: unset old cluster version in metrics

rnaveiras/etcd,coreos/etcd,ajityagaty/etcd,li-ang/etcd,rnaveiras/etcd,etcd-io/etcd,mqliang/etcd,mqliang/etcd,gyuho/etcd,li-ang/etcd,mkumatag/etcd,cockroachdb/etcd,etcd-io/etcd,gyuho/etcd,cockroachdb/etcd,dannysauer/etcd,ajityagaty/etcd,etcd-io/etcd,mkumatag/etcd,ajityagaty/etcd,mkumatag/etcd,coreos/etcd,dannysauer/etcd,rnaveiras/etcd,mqliang/etcd