code: @BetaApi
    public final ListInstancesPagedResponse listInstances(String zone) {
      ListInstancesHttpRequest request = ListInstancesHttpRequest.newBuilder().setZone(zone).build();
      return listInstances(request);
    }
docs: Retrieves the list of instances contained within the specified zone.
  
  <p>Sample code:
  
  <pre><code>
  try (InstanceClient instanceClient = InstanceClient.create()) {
  ProjectZoneName zone = ProjectZoneName.of("[PROJECT]", "[ZONE]");
  for (Instance element : instanceClient.listInstances(zone.toString()).iterateAll()) {
  // doThingsWith(element);
  }
  }
  </code></pre>
  
  @param zone The name of the zone for this request.
  @throws com.google.api.gax.rpc.ApiException if the remote call fails