private void ensureNotStarted() {
        if (isStarted()) {
            throw new IllegalStateException("can't update the export list once started");
        }
    }
Adds the export group.
Note: this method is meant to be used for XML configuration.
logback/logback12/src/main/java/com/linecorp/armeria/common/logback/RequestContextExportingAppender.java