@Override
                                public void close(Status status, Metadata trailers) {
                                    trailers.merge(requestHeaders);
                                    super.close(status, trailers);
                                }
Must be at least {@link #unaryPayloadLength()}, plus some to account for encoding overhead.
grpc/src/test/java/com/linecorp/armeria/client/grpc/GrpcClientTest.java