code: @BetaApi
    public final Operation patchRegionInstanceGroupManager(
        String instanceGroupManager,
        InstanceGroupManager instanceGroupManagerResource,
        List<String> fieldMask) {
  
      PatchRegionInstanceGroupManagerHttpRequest request =
          PatchRegionInstanceGroupManagerHttpRequest.newBuilder()
              .setInstanceGroupManager(instanceGroupManager)
              .setInstanceGroupManagerResource(instanceGroupManagerResource)
              .addAllFieldMask(fieldMask)
              .build();
      return patchRegionInstanceGroupManager(request);
    }
docs: Updates a managed instance group using the information that you specify in the request. This
  operation is marked as DONE when the group is patched even if the instances in the group are
  still in the process of being patched. You must separately verify the status of the individual
  instances with the listmanagedinstances method. This method supports PATCH semantics and uses
  the JSON merge patch format and processing rules.
  
  <p>Sample code:
  
  <pre><code>
  try (RegionInstanceGroupManagerClient regionInstanceGroupManagerClient = RegionInstanceGroupManagerClient.create()) {
  ProjectRegionInstanceGroupManagerName instanceGroupManager = ProjectRegionInstanceGroupManagerName.of("[PROJECT]", "[REGION]", "[INSTANCE_GROUP_MANAGER]");
  InstanceGroupManager instanceGroupManagerResource = InstanceGroupManager.newBuilder().build();
  List&lt;String&gt; fieldMask = new ArrayList&lt;&gt;();
  Operation response = regionInstanceGroupManagerClient.patchRegionInstanceGroupManager(instanceGroupManager.toString(), instanceGroupManagerResource, fieldMask);
  }
  </code></pre>
  
  @param instanceGroupManager The name of the instance group manager.
  @param instanceGroupManagerResource An Instance Group Manager resource. (== resource_for
  beta.instanceGroupManagers ==) (== resource_for v1.instanceGroupManagers ==) (==
  resource_for beta.regionInstanceGroupManagers ==) (== resource_for
  v1.regionInstanceGroupManagers ==)
  @param fieldMask The fields that should be serialized (even if they have empty values). If the
  containing message object has a non-null fieldmask, then all the fields in the field mask
  (and only those fields in the field mask) will be serialized. If the containing object does
  not have a fieldmask, then only non-empty fields will be serialized.
  @throws com.google.api.gax.rpc.ApiException if the remote call fails