Metrics example 404s - fix url
The metrics example link points to a file that appears to have moved. This change points it to what I think is the right place.
@@ -77,7 +77,7 @@ 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/etcd-io/etcd/blob/master/clientv3/example_metrics_test.go).
+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/etcd-io/etcd/blob/master/tests/integration/clientv3/examples/example_metrics_test.go).
## Namespacing
