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:
    
    void signalReadyToRun();
    BackgroundProcessingPoolTaskInfo(BackgroundProcessingPool & pool_, const BackgroundProcessingPool::Task & function_)
        : pool(pool_), task_function(function_) {}
protected:
    friend class BackgroundProcessingPool;
    BackgroundProcessingPool & pool;
    BackgroundProcessingPool::Task task_function;
    
    
    std::shared_mutex rwlock;
    bool allow_execute_in_parallel = false;
    size_t concurrent_executors = 0;
    
    std::atomic<bool> removed {false};
    BackgroundProcessingPool::Tasks::iterator iterator;
    
    size_t count_no_work_done = 0;
};
}