@Override
                public <REQ, RESP> Listener<REQ> interceptCall(
                        ServerCall<REQ, RESP> call,
                        Metadata requestHeaders,
                        ServerCallHandler<REQ, RESP> next) {
                    final HttpHeadersBuilder fromClient = HttpHeaders.builder();
                    MetadataUtil.fillHeaders(requestHeaders, fromClient);
                    CLIENT_HEADERS_CAPTURE.set(fromClient.build());
                    return next.startCall(
                            new SimpleForwardingServerCall<REQ, RESP>(call) {
                                @Override
                                public void close(Status status, Metadata trailers) {
                                    trailers.merge(requestHeaders);
                                    super.close(status, trailers);
                                }
                            }, requestHeaders);
                }
Must be at least {@link #unaryPayloadLength()}, plus some to account for encoding overhead.
grpc/src/test/java/com/linecorp/armeria/client/grpc/GrpcClientTest.java