String xpath = configMap.get(Config.HTML_CANONICAL_XPATH);
        if (xpath == null) {
            xpath = fessConfig.getCrawlerDocumentHtmlCanonicalXpath();
        }
        if (StringUtil.isBlank(xpath)) {
            return null;
        }
        final String canonicalUrl = getSingleNodeValue(document, xpath, node -> node);

     * comment: XPath to extract canonical URL from HTML documents.
     * @return The value of found property. (NotNull: if not found, exception but basically no way)
     */
    String getCrawlerDocumentHtmlCanonicalXpath();

    /**
     * Get the value for the key 'crawler.document.html.pruned.tags'. <br>
     * The value is, e.g. noscript,script,style,header,footer,aside,nav,a[rel=nofollow] <br>