tools/etcd-dump-metrics: add missing godoc

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

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

tools/etcd-dump-metrics/main.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// etcd-dump-metrics automates etcd Prometheus metrics documentation.
 package main
 
 import (