e2e: test /metrics, /health endpoint in grpc-proxy
e2e: test /metrics, /health endpoint in grpc-proxy

Signed-off-by: Gyu-Ho Lee <3454051ee3341eb69bdf6fca40899cdad4d86496@gmail.com>

etcd-io/etcd,rnaveiras/etcd,bdarnell/etcd,etcd-io/etcd,mkumatag/etcd,li-ang/etcd,gyuho/etcd,gyuho/etcd,sinsharat/etcd,xiang90/etcd,visheshnp/etcd,sinsharat/etcd,xiang90/etcd,lorneli/etcd,ajityagaty/etcd,etcd-io/etcd,li-ang/etcd,cockroachdb/etcd,a-robinson/etcd,vimalk78/etcd,whuwxl/etcd,visheshnp/etcd,whuwxl/etcd,heyitsanthony/etcd,akauppi/etcd,glevand/coreos--etcd,sebrandon1/etcd,mickep76/etcd,ajityagaty/etcd,mickep76/etcd,bdarnell/etcd,mqliang/etcd,YuleiXiao/etcd,rnaveiras/etcd,mqliang/etcd,vimalk78/etcd,fanminshi/etcd,joshix/etcd,sebrandon1/etcd,cockroachdb/etcd,dannysauer/etcd,a-robinson/etcd,fanminshi/etcd,YuleiXiao/etcd,coreos/etcd,coreos/etcd,ajityagaty/etcd,mkumatag/etcd,dannysauer/etcd,mkumatag/etcd,lorneli/etcd,bluebreezecf/etcd,bluebreezecf/etcd,akauppi/etcd,mqliang/etcd,heyitsanthony/etcd,glevand/coreos--etcd,rnaveiras/etcd,joshix/etcd