@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