code: @BetaApi
    public final Operation deleteInstancesInstanceGroupManager(
        String instanceGroupManager,
        InstanceGroupManagersDeleteInstancesRequest
            instanceGroupManagersDeleteInstancesRequestResource) {
  
      DeleteInstancesInstanceGroupManagerHttpRequest request =
          DeleteInstancesInstanceGroupManagerHttpRequest.newBuilder()
              .setInstanceGroupManager(instanceGroupManager)
              .setInstanceGroupManagersDeleteInstancesRequestResource(
                  instanceGroupManagersDeleteInstancesRequestResource)
              .build();
      return deleteInstancesInstanceGroupManager(request);
    }
docs: Flags the specified instances in the managed instance group for immediate deletion. The
  instances are also removed from any target pools of which they were a member. This method
  reduces the targetSize of the managed instance group by the number of instances that you
  delete. This operation is marked as DONE when the action is scheduled even if the instances are
  still being deleted. You must separately verify the status of the deleting action with the
  listmanagedinstances method.
  
  <p>If the group is part of a backend service that has enabled connection draining, it can take
  up to 60 seconds after the connection draining duration has elapsed before the VM instance is
  removed or deleted.
  
  <p>You can specify a maximum of 1000 instances with this method per request.
  
  <p>Sample code:
  
  <pre><code>
  try (InstanceGroupManagerClient instanceGroupManagerClient = InstanceGroupManagerClient.create()) {
  ProjectZoneInstanceGroupManagerName instanceGroupManager = ProjectZoneInstanceGroupManagerName.of("[PROJECT]", "[ZONE]", "[INSTANCE_GROUP_MANAGER]");
  InstanceGroupManagersDeleteInstancesRequest instanceGroupManagersDeleteInstancesRequestResource = InstanceGroupManagersDeleteInstancesRequest.newBuilder().build();
  Operation response = instanceGroupManagerClient.deleteInstancesInstanceGroupManager(instanceGroupManager.toString(), instanceGroupManagersDeleteInstancesRequestResource);
  }
  </code></pre>
  
  @param instanceGroupManager The name of the managed instance group.
  @param instanceGroupManagersDeleteInstancesRequestResource
  @throws com.google.api.gax.rpc.ApiException if the remote call fails