@Nullable
    private static String getResponseBody(List<FieldDescriptor> topLevelFields,
                                          @Nullable String responseBody) {
        if (StringUtil.isNullOrEmpty(responseBody)) {
            return null;
        }
        for (FieldDescriptor fieldDescriptor : topLevelFields) {
            if (fieldDescriptor.getName().equals(responseBody)) {
                return responseBody;
            }
        }
        return null;
    }
Creates a new {@link GrpcService} instance from the given {@code delegate}. If it is possible
to support HTTP/JSON to gRPC transcoding, a new {@link HttpJsonTranscodingService} instance
would be returned. Otherwise, the {@code delegate} would be returned.
grpc/src/main/java/com/linecorp/armeria/server/grpc/HttpJsonTranscodingService.java