@Override
    public String getFetchKey() {
        return fetchKey;
    }
Basic class of multi topic fetchers.
The main differences between this and {@link SingleTopicFetcher} is that:
1. MultiTopicFetcher maintains a list of topics while {@link SingleTopicFetcher} only maintains one;
2. All topics share the same properties;
3. The joining and removing of topics will result in new creation of consumer,
     and the old ones will be put in a list, waiting to be cleaned by a scheduled thread.
inlong-sdk/sort-sdk/src/main/java/org/apache/inlong/sdk/sort/api/MultiTopicsFetcher.java