public String putCollection(final String key, Collection<String> values) {
        final String value;
        if (values == null) {
            value = null;
        } else if (values.isEmpty()) {
            value = "";
        } else {
            value = String.join(VALUE_DELIMITER, values);
        }
        return put(key, value);
    }
Put an entry where the value is itself a collection of values, e.g. a list of files
stroom-meta/stroom-meta-api/src/main/java/stroom/meta/api/AttributeMap.java