embed: split peer/client/metrics serve methods
embed: split peer/client/metrics serve methods

Priliminary commit to start client server later.

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

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