code: @BetaApi
    public final Operation patchRegionBackendService(
        String backendService, BackendService backendServiceResource, List<String> fieldMask) {
  
      PatchRegionBackendServiceHttpRequest request =
          PatchRegionBackendServiceHttpRequest.newBuilder()
              .setBackendService(backendService)
              .setBackendServiceResource(backendServiceResource)
              .addAllFieldMask(fieldMask)
              .build();
      return patchRegionBackendService(request);
    }
docs: Updates the specified regional BackendService resource with the data included in the request.
  There are several restrictions and guidelines to keep in mind when updating a backend service.
  Read Restrictions and Guidelines for more information. This method supports PATCH semantics and
  uses the JSON merge patch format and processing rules.
  
  <p>Sample code:
  
  <pre><code>
  try (RegionBackendServiceClient regionBackendServiceClient = RegionBackendServiceClient.create()) {
  ProjectRegionBackendServiceName backendService = ProjectRegionBackendServiceName.of("[PROJECT]", "[REGION]", "[BACKEND_SERVICE]");
  BackendService backendServiceResource = BackendService.newBuilder().build();
  List&lt;String&gt; fieldMask = new ArrayList&lt;&gt;();
  Operation response = regionBackendServiceClient.patchRegionBackendService(backendService.toString(), backendServiceResource, fieldMask);
  }
  </code></pre>
  
  @param backendService Name of the BackendService resource to patch.
  @param backendServiceResource A BackendService resource. This resource defines a group of
  backend virtual machines and their serving capacity. (== resource_for v1.backendService ==)
  (== resource_for beta.backendService ==)
  @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