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:
    BackgroundSchedulePoolTaskInfo(BackgroundSchedulePool & pool_, const std::string & log_name_, const BackgroundSchedulePool::TaskFunc & function_);
    
    
    bool schedule();
    
    
    bool scheduleAfter(size_t ms, bool overwrite = true);
    
    void deactivate();
    void activate();
    
    bool activateAndSchedule();
    
    Coordination::WatchCallback getWatchCallback();
private:
    friend class TaskNotification;
    friend class BackgroundSchedulePool;
    void execute();
    void scheduleImpl(std::lock_guard<std::mutex> & schedule_mutex_lock);
    BackgroundSchedulePool & pool;
    std::string log_name;
    BackgroundSchedulePool::TaskFunc function;
    std::mutex exec_mutex;
    std::mutex schedule_mutex;