When I queried `system.metrics` table, I found some negative values which hint to some potential internal overflows.

```
SELECT *
FROM system.metrics
FORMAT Vertical

...

Row 31:
───────
metric:      MemoryTrackingInBackgroundProcessingPool
value:       -580863171
description: Total amount of memory (bytes) allocated in background processing pool (that is dedicated for backround merges, mutations and fetches). Note that this value may include a drift when the memory was allocated in a context of background processing pool and freed in other context or vice-versa. This happens naturally due to caches for tables indexes and doesn't indicate memory leaks.

Row 32:
───────
metric:      MemoryTrackingInBackgroundMoveProcessingPool
value:       0
description: Total amount of memory (bytes) allocated in background processing pool (that is dedicated for backround moves). Note that this value may include a drift when the memory was allocated in a context of background processing pool and freed in other context or vice-versa. This happens naturally due to caches for tables indexes and doesn't indicate memory leaks.

Row 33:
───────
metric:      MemoryTrackingInBackgroundSchedulePool
value:       -4461308831
description: Total amount of memory (bytes) allocated in background schedule pool (that is dedicated for bookkeeping tasks of Replicated tables).

...

56 rows in set. Elapsed: 0.003 sec.
```

Server version: 20.8.2.3.
{
    scheduled = true;
    if (delayed)
        pool.cancelDelayedTask(shared_from_this(), schedule_mutex_lock);
    
    
    
    if (!executing)
        pool.queue.enqueueNotification(new TaskNotification(shared_from_this()));
}
Coordination::WatchCallback BackgroundSchedulePoolTaskInfo::getWatchCallback()
{
     return [t = shared_from_this()](const Coordination::WatchResponse &)
     {
         t->schedule();
     };
}
BackgroundSchedulePool::BackgroundSchedulePool(size_t size_, CurrentMetrics::Metric tasks_metric_, CurrentMetrics::Metric memory_metric_, const char *thread_name_)
    : size(size_)
    , tasks_metric(tasks_metric_)
    , memory_metric(memory_metric_)
    , thread_name(thread_name_)
{
    LOG_INFO(&Poco::Logger::get("BackgroundSchedulePool/" + thread_name), "Create BackgroundSchedulePool with {} threads", size);
    threads.resize(size);
    for (auto & thread : threads)
        thread = ThreadFromGlobalPool([this] { threadFunction(); });
    delayed_thread = ThreadFromGlobalPool([this] { delayExecutionThreadFunction(); });
}
BackgroundSchedulePool::~BackgroundSchedulePool()