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.
}
        else
        {
            CurrentThread::initializeQuery();
            thread_group = CurrentThread::getGroup();
        }
    }
    SCOPE_EXIT({ CurrentThread::detachQueryIfNotDetached(); });
    if (auto * const memory_tracker = CurrentThread::getMemoryTracker())
        memory_tracker->setMetric(settings.memory_metric);
    pcg64 rng(randomSeed());
    std::this_thread::sleep_for(std::chrono::duration<double>(std::uniform_real_distribution<double>(0, settings.thread_sleep_seconds_random_part)(rng)));
    Poco::Timestamp scheduled_task_start_time;
    while (true)
    {
        TaskResult task_result = TaskResult::ERROR;
        TaskHandle task;
        {
            std::unique_lock lock(tasks_mutex);
            while (!task && !shutdown)
            {
                for (const auto & [time, handle] : tasks)
                {
                    if (!handle->removed
                        && (handle->allow_execute_in_parallel || handle->concurrent_executors == 0))
                    {
                        task = handle;
                        scheduled_task_start_time = time;
                        ++task->concurrent_executors;
                        break;