constructor(method, requestHeaders, request, headers, response, status, trailers) {
        this.method = method;
        this.requestHeaders = requestHeaders;
        this.request = request;
        this.headers = headers;
        this.response = response;
        this.status = status;
        this.trailers = trailers;
    }
A unary RPC call. Unary means there is exactly one input message and
exactly one output message unless an error occurred.