@Override
                                       public void applyRequestMetadata(RequestInfo requestInfo,
                                                                        Executor appExecutor,
                                                                        MetadataApplier applier) {
                                           assertThat(requestInfo.getMethodDescriptor())
                                                   .isEqualTo(TestServiceGrpc.getEmptyCallMethod());
                                           assertThat(requestInfo.getAuthority())
                                                   .isEqualTo("localhost:" + server.httpPort());
                                           assertThat(requestInfo.getSecurityLevel())
                                                   .isEqualTo(SecurityLevel.NONE);
                                           assertThat(appExecutor).isEqualTo(
                                                   CommonPools.blockingTaskExecutor());

                                           CommonPools.blockingTaskExecutor().schedule(() -> {
                                               final Metadata metadata = new Metadata();
                                               metadata.put(TestServiceImpl.EXTRA_HEADER_KEY, "token");
                                               applier.apply(metadata);
                                           }, 100, TimeUnit.MILLISECONDS);
                                       }
Must be at least {@link #unaryPayloadLength()}, plus some to account for encoding overhead.
grpc/src/test/java/com/linecorp/armeria/client/grpc/GrpcClientTest.java