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.
{
    Stopwatch watch;
    CurrentMetrics::Increment metric_increment{pool.tasks_metric};
    std::lock_guard lock_exec(exec_mutex);
    {
        std::lock_guard lock_schedule(schedule_mutex);
        if (deactivated)
            return;
        scheduled = false;
        executing = true;
    }
    function();
    UInt64 milliseconds = watch.elapsedMilliseconds();
    
    static const int32_t slow_execution_threshold_ms = 200;
    if (milliseconds >= slow_execution_threshold_ms)
        LOG_TRACE(&Poco::Logger::get(log_name), "Execution took {} ms.", milliseconds);
    {
        std::lock_guard lock_schedule(schedule_mutex);
        executing = false;
        
        
        
        if (scheduled)
            pool.queue.enqueueNotification(new TaskNotification(shared_from_this()));
    }
}
void BackgroundSchedulePoolTaskInfo::scheduleImpl(std::lock_guard<std::mutex> & schedule_mutex_lock)