code: @BetaApi
    public final ListMachineTypesPagedResponse listMachineTypes(ProjectZoneName zone) {
      ListMachineTypesHttpRequest request =
          ListMachineTypesHttpRequest.newBuilder()
              .setZone(zone == null ? null : zone.toString())
              .build();
      return listMachineTypes(request);
    }
docs: Retrieves a list of machine types available to the specified project.
  
  <p>Sample code:
  
  <pre><code>
  try (MachineTypeClient machineTypeClient = MachineTypeClient.create()) {
  ProjectZoneName zone = ProjectZoneName.of("[PROJECT]", "[ZONE]");
  for (MachineType element : machineTypeClient.listMachineTypes(zone).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