private static Map<String, String> extractMdc(Map<String, String> map, List<String> keys) {
    return keys.stream()
      .filter(key -> isNotBlank(map.getOrDefault(key, ""))
      )
      .collect(Collectors.toMap(key -> key, map::get));
  }
Get list of fields and populate values into map (if value for key is not empty

@param map  map to fill
@param keys list of xpaths in json files
@return map populated by additional xpath values
olv-core/src/main/java/pl/otros/logview/parser/json/JsonExtractor.java