private Map<String, String> prepareMdcMap(Map<String, String> contextMap,
                                              Map<String, String> originalMdcMap) {
        if (needsHashMap) {
            final Map<String, String> mdcMap = new HashMap<>(contextMap);
            mdcMap.putAll(originalMdcMap);
            return mdcMap;
        }
        if (!originalMdcMap.isEmpty()) {
            return new UnionMap<>(contextMap, originalMdcMap);
        }
        return contextMap;
    }
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