DDLWorker thread checks in a loop for new tasks by calling ZooKeeper GetChildren with a watch callback.

https://github.com/ClickHouse/ClickHouse/blob/5b0bc8a7fbf846cb45012e8009d58341656bf12c/src/Interpreters/DDLWorker.cpp#L374

Due to a recent change from #25373 instead of blocking on new events we retry the loop every 10 seconds instead of when watch callback is triggered.

https://github.com/ClickHouse/ClickHouse/blob/5b0bc8a7fbf846cb45012e8009d58341656bf12c/src/Interpreters/DDLWorker.cpp#L1140-L1142

This causes a zookeeper watch callback leak. It is probably not very important as all that happens is that a new std::function is added to the watches list for the ddl worker path every 10 seconds. What is 32 bytes (sizeof(std::function)?) every 10 seconds?

However, it is annoying to see that on metrics. The metric is somewhat useful for catching bugs and this isn't an important one.

I understand that we could probably fix this "temporary fix" in DDLWorker and remove the tryWait + retrying, but that pattern was seen in other places and is useful for implementing a non-blocking watch, also used in `waitForTableReplicaToProcessLogEntry`.

Is there a laconic fix for that? Maybe we could somehow remove the callback when not needed/before retry? Maybe we could change the behaviour slightly and add watchcallback only if it wasn't already added (this wouldn't work for getChildren which wraps a Poco Event in a watch)? Anything else beside just ignoring this?

---

I understand that all the callbacks will be cleared when the event is triggered.
*  are atomically checking that the lock still holds
  *  or if we ensure that these operations will be undone if lock is lost
  *  (due to ZooKeeper session loss) that's very difficult to achieve.
  *
  * It's Ok if every operation that we perform under lock is actually operation in ZooKeeper.
  *
  * In 1% of cases when you can correctly use Lock, the logic is complex enough, so you don't need this class.
  *
  * TLDR: Don't use this code.
  * We only have a few cases of it's usage and it will be removed.
  */
class ZooKeeperLock
{
public:
    
    
    ZooKeeperLock(
        const zkutil::ZooKeeperPtr & zookeeper_,
        const std::string & lock_prefix_,
        const std::string & lock_name_,
        const std::string & lock_message_ = "")
    :
        zookeeper(zookeeper_),
        lock_path(fs::path(lock_prefix_) / lock_name_),
        lock_message(lock_message_),
        log(&Poco::Logger::get("zkutil::Lock"))
    {
        zookeeper->createIfNotExists(lock_prefix_, "");
    }
    ~ZooKeeperLock()