clientv3: updated doc for metric support

clientv3/README.md

@@ -72,6 +72,10 @@ if err != nil {
 }
 ```
 
+## Metrics
+
+The etcd client optionally exposes RPC metrics through [go-grpc-prometheus](https://github.com/grpc-ecosystem/go-grpc-prometheus). See the [examples](https://github.com/coreos/etcd/blob/master/clientv3/example_metrics_test.go).
+
 ## Examples
 
 More code examples can be found at [GoDoc](https://godoc.org/github.com/coreos/etcd/clientv3).