private Map<String, Object> diffFields(Fields fields) {
            return this.properties.entrySet().stream()
                                  .filter(e -> !fields.properties.containsKey(e.getKey()))
                                  .collect(Collectors.toMap(
                                      Map.Entry::getKey,
                                      Map.Entry::getValue
                                  ));
        }
Returns the properties that not exist in the input fields.
oap-server/server-storage-plugin/storage-elasticsearch-plugin/src/main/java/org/apache/skywalking/oap/server/storage/plugin/elasticsearch/base/IndexStructures.java