@Override
            public void applyRequestMetadata(RequestInfo requestInfo, Executor appExecutor,
                                             MetadataApplier applier) {
                final Metadata metadata = new Metadata();
                metadata.put(TestServiceImpl.EXTRA_HEADER_KEY, "token");
                finishCredentials.thenAccept(unused -> 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