unaryCall(method, requestMessage, metadata, methodDescriptor, options = {}) {
    return /** @type {!Promise<RESPONSE>}*/ (this.thenableCall(
        method, requestMessage, metadata, methodDescriptor, options));
  }
@export
@param {string} method The method to invoke
@param {REQUEST} requestMessage The request proto
@param {!Object<string, string>} metadata User defined call metadata
@param {!MethodDescriptor<REQUEST, RESPONSE>} methodDescriptor Information
    of this RPC method
@param {?PromiseCallOptions=} options Options for the call
@return {!Promise<RESPONSE>}
@template REQUEST, RESPONSE