tools/etcd-dump-metrics: add missing godoc
tools/etcd-dump-metrics: add missing godoc

https://godoc.org/github.com/etcd-io/etcd/tools/etcd-dump-metrics

Signed-off-by: Gyuho Lee <0fbd30d364d71722b926950b451cecd97b54fc29@amazon.com>

mqliang/etcd,li-ang/etcd,mqliang/etcd,cockroachdb/etcd,mkumatag/etcd,etcd-io/etcd,dannysauer/etcd,xiang90/etcd,etcd-io/etcd,mkumatag/etcd,joshix/etcd,ajityagaty/etcd,xiang90/etcd,coreos/etcd,rnaveiras/etcd,li-ang/etcd,gyuho/etcd,ajityagaty/etcd,mqliang/etcd,coreos/etcd,cockroachdb/etcd,dannysauer/etcd,rnaveiras/etcd,etcd-io/etcd,rnaveiras/etcd,joshix/etcd,gyuho/etcd,ajityagaty/etcd,mkumatag/etcd