code: @BetaApi
    public final ListTargetInstancesPagedResponse listTargetInstances(String zone) {
      ListTargetInstancesHttpRequest request =
          ListTargetInstancesHttpRequest.newBuilder().setZone(zone).build();
      return listTargetInstances(request);
    }
docs: Retrieves a list of TargetInstance resources available to the specified project and zone.
  
  <p>Sample code:
  
  <pre><code>
  try (TargetInstanceClient targetInstanceClient = TargetInstanceClient.create()) {
  ProjectZoneName zone = ProjectZoneName.of("[PROJECT]", "[ZONE]");
  for (TargetInstance element : targetInstanceClient.listTargetInstances(zone.toString()).iterateAll()) {
  // doThingsWith(element);
  }
  }
  </code></pre>
  
  @param zone Name of the zone scoping this request.
  @throws com.google.api.gax.rpc.ApiException if the remote call fails