code: function _buildChain(interceptors, options) {
    var next = function(interceptors) {
      if (interceptors.length === 0) {
        return function (options) {};
      }
      var head_interceptor = interceptors[0];
      var rest_interceptors = interceptors.slice(1);
      return function (options) {
        return head_interceptor(options, next(rest_interceptors));
      };
    };
    var chain = next(interceptors)(options);
    return new InterceptingCall(chain);
  }
docs: Chain a list of interceptors together and return the first InterceptingCall.
  @private
  @param {Interceptor[]} interceptors An interceptor stack.
  @param {grpc.Client~CallOptions} options Call options.
  @return {InterceptingCall}