@Override
    protected void append(ILoggingEvent eventObject) {
        if (exporter == null) {
            if (!FlagsLoaded.get()) {
                // It is possible that requestContextStorageProvider hasn't been initialized yet
                // due to static variable circular dependency.
                // Most notably, this can happen when logs are appended while trying to initialize
                // Flags#requestContextStorageProvider.
                aai.appendLoopOnAppenders(eventObject);
                return;
            }
            // Build the exporter lazily to prevent the customizer from initializing Flags.
            // See: https://github.com/line/armeria/issues/5327
            final RequestContextExporterBuilder builder = RequestContextExporter.builder();
            customizer.accept(builder);
            exporter = builder.build();
        }
        final Map<String, String> contextMap = exporter.export();
        if (!contextMap.isEmpty()) {
            final Map<String, String> originalMdcMap = eventObject.getMDCPropertyMap();
            final Map<String, String> mdcMap = prepareMdcMap(contextMap, originalMdcMap);
            eventObject = new LoggingEventWrapper(eventObject, mdcMap);
        }
        aai.appendLoopOnAppenders(eventObject);
    }
Stores the specified {@link RequestContextExporterBuilder} customizer to be lazily applied when
the exporter is built.
logback/logback12/src/main/java/com/linecorp/armeria/common/logback/RequestContextExportingAppender.java