private static void addMetadata(Metadata m, Property p, String value) {
        if (value != null) {
            if (p.isMultiValuePermitted() || m.get(p) == null) {
                m.add(p, value);
            }
        }
    }
Extracts Media Management metadata from XMP.
<p>
Silently swallows exceptions.

@param xmp
@param metadata
tika-parsers/tika-parsers-standard/tika-parsers-standard-modules/tika-parser-xmp-commons/src/main/java/org/apache/tika/parser/xmp/JempboxExtractor.java