constructor(requestMessage, methodDescriptor, metadata, callOptions) {
    /**
     * @const {REQUEST}
     * @private
     */
    this.requestMessage_ = requestMessage;

    /**
     * @const {!MethodDescriptor<REQUEST, RESPONSE>}
     * @private
     */
    this.methodDescriptor_ = methodDescriptor;

    /** @const @private */
    this.metadata_ = metadata;

    /** @const @private */
    this.callOptions_ = callOptions;
  }
@param {REQUEST} requestMessage
@param {!MethodDescriptor<REQUEST, RESPONSE>} methodDescriptor
@param {!Metadata} metadata
@param {!CallOptions} callOptions