etcdctl/ctlv3/command: don't print gRPC WARNING logs by default
etcdctl/ctlv3/command: don't print gRPC WARNING logs by default

Despite holding important information, these are too noisy.

See https://github.com/coreos/etcd/pull/9623

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