How to use the @google-cloud/common.Service.call function in @google-cloud/common

function GrpcService(config, options) {
  if (global.GCLOUD_SANDBOX_ENV) {
    // gRPC has a tendency to cause our doc unit tests to fail, so we prevent
    // any calls to that library from going through.
    // Reference: https://github.com/GoogleCloudPlatform/google-cloud-node/pull/1137#issuecomment-193315047
    return global.GCLOUD_SANDBOX_ENV;
  }

  Service.call(this, config, options);

  if (config.customEndpoint) {
    this.grpcCredentials = grpc.credentials.createInsecure();
  }

  this.grpcMetadata = new grpc.Metadata();

  this.grpcMetadata.add('x-goog-api-client', [
    'gl-node/' + process.versions.node,
    'gccl/' + config.packageJson.version,
    'grpc/' + require('grpc/package.json').version
  ].join(' '));

  if (config.grpcMetadata) {
    for (var prop in config.grpcMetadata) {
      if (config.grpcMetadata.hasOwnProperty(prop)) {

function GrpcService(config, options) {
  if (global.GCLOUD_SANDBOX_ENV) {
    // gRPC has a tendency to cause our doc unit tests to fail, so we prevent
    // any calls to that library from going through.
    // Reference: https://github.com/GoogleCloudPlatform/google-cloud-node/pull/1137#issuecomment-193315047
    return global.GCLOUD_SANDBOX_ENV;
  }

  Service.call(this, config, options);

  if (config.customEndpoint) {
    this.grpcCredentials = grpc.credentials.createInsecure();
  }

  this.grpcMetadata = new grpc.Metadata();

  this.grpcMetadata.add('x-goog-api-client', [
    'gl-node/' + process.versions.node,
    'gccl/' + config.packageJson.version,
    'grpc/' + require('grpc/package.json').version
  ].join(' '));

  if (config.grpcMetadata) {
    for (var prop in config.grpcMetadata) {
      if (config.grpcMetadata.hasOwnProperty(prop)) {