code: @BetaApi
    public final Operation deleteInstancesRegionInstanceGroupManager(
        ProjectRegionInstanceGroupManagerName instanceGroupManager,
        RegionInstanceGroupManagersDeleteInstancesRequest
            regionInstanceGroupManagersDeleteInstancesRequestResource) {
  
      DeleteInstancesRegionInstanceGroupManagerHttpRequest request =
          DeleteInstancesRegionInstanceGroupManagerHttpRequest.newBuilder()
              .setInstanceGroupManager(
                  instanceGroupManager == null ? null : instanceGroupManager.toString())
              .setRegionInstanceGroupManagersDeleteInstancesRequestResource(
                  regionInstanceGroupManagersDeleteInstancesRequestResource)
              .build();
      return deleteInstancesRegionInstanceGroupManager(request);
    }
docs: Flags the specified instances in the managed instance group to be immediately deleted. 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. The deleteInstances operation is marked DONE if the deleteInstances request is
  successful. The underlying actions take additional time. 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 (RegionInstanceGroupManagerClient regionInstanceGroupManagerClient = RegionInstanceGroupManagerClient.create()) {
  ProjectRegionInstanceGroupManagerName instanceGroupManager = ProjectRegionInstanceGroupManagerName.of("[PROJECT]", "[REGION]", "[INSTANCE_GROUP_MANAGER]");
  RegionInstanceGroupManagersDeleteInstancesRequest regionInstanceGroupManagersDeleteInstancesRequestResource = RegionInstanceGroupManagersDeleteInstancesRequest.newBuilder().build();
  Operation response = regionInstanceGroupManagerClient.deleteInstancesRegionInstanceGroupManager(instanceGroupManager, regionInstanceGroupManagersDeleteInstancesRequestResource);
  }
  </code></pre>
  
  @param instanceGroupManager Name of the managed instance group.
  @param regionInstanceGroupManagersDeleteInstancesRequestResource
  @throws com.google.api.gax.rpc.ApiException if the remote call fails