How to use the google-gax.BundleDescriptor function in google-gax

var protoFilesRoot = new gax.GoogleProtoFilesRoot();
    protoFilesRoot = protobuf.loadSync(
      path.join(
        __dirname,
        '..',
        '..',
        'protos',
        'google/logging/v2/logging.proto'
      ),
      protoFilesRoot
    );

    // Some methods on this API support automatically batching
    // requests; denote this.
    this._descriptors.batching = {
      writeLogEntries: new gax.BundleDescriptor(
        'entries',
        ['logName', 'resource', 'labels'],
        null,
        gax.createByteLengthFunction(
          protoFilesRoot.lookup('google.logging.v2.LogEntry')
        )
      ),
    };

    // Put together the default options sent with requests.
    var defaults = gaxGrpc.constructSettings(
      'google.logging.v2.LoggingServiceV2',
      gapicConfig,
      opts.clientConfig,
      {'x-goog-api-client': clientHeader.join(' ')}
    );