@Override
                    public void applyRequestMetadata(RequestInfo requestInfo,
                                                     Executor appExecutor,
                                                     MetadataApplier applier) {
                        final Metadata metadata = new Metadata();
                        metadata.put(TestServiceImpl.EXTRA_HEADER_KEY, "token2");
                        applier.apply(metadata);
                    }
Must be at least {@link #unaryPayloadLength()}, plus some to account for encoding overhead.
grpc/src/test/java/com/linecorp/armeria/client/grpc/GrpcClientTest.java