private void setMessageParameters(DocumentOperationMessageV3 msg, FeederSettings settings) {
        msg.getMessage().setContext(new ReplyContext(msg.getOperationId(), feedReplies));
        if (settings.traceLevel != null) {
            msg.getMessage().getTrace().setLevel(settings.traceLevel);
        }
    }
Returns the next message in the stream, or null if none
vespaclient-container-plugin/src/main/java/com/yahoo/vespa/http/server/ClientFeederV3.java