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.
{
public:
    
    using TaskResult = BackgroundProcessingPoolTaskResult;
    using Task = std::function<TaskResult()>;
    using TaskInfo = BackgroundProcessingPoolTaskInfo;
    using TaskHandle = std::shared_ptr<TaskInfo>;
    struct PoolSettings
    {
        double thread_sleep_seconds = 10;
        double thread_sleep_seconds_random_part = 1.0;
        double thread_sleep_seconds_if_nothing_to_do = 0.1;
        
        double task_sleep_seconds_when_no_work_min = 10;
        double task_sleep_seconds_when_no_work_max = 600;
        double task_sleep_seconds_when_no_work_multiplier = 1.1;
        double task_sleep_seconds_when_no_work_random_part = 1.0;
        CurrentMetrics::Metric tasks_metric = CurrentMetrics::BackgroundPoolTask;
        CurrentMetrics::Metric memory_metric = CurrentMetrics::MemoryTrackingInBackgroundProcessingPool;
        PoolSettings() noexcept {}
    };
    BackgroundProcessingPool(int size_,
        const PoolSettings & pool_settings = {},
        const char * log_name = "BackgroundProcessingPool",
        const char * thread_name_ = "BackgrProcPool");
    size_t getNumberOfThreads() const