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.
BackgroundSchedulePool(size_t size_, CurrentMetrics::Metric tasks_metric_, CurrentMetrics::Metric memory_metric_, const char *thread_name_);
    ~BackgroundSchedulePool();
private:
    using Threads = std::vector<ThreadFromGlobalPool>;
    void threadFunction();
    void delayExecutionThreadFunction();
    
    void scheduleDelayedTask(const TaskInfoPtr & task_info, size_t ms, std::lock_guard<std::mutex> & task_schedule_mutex_lock);
    
    void cancelDelayedTask(const TaskInfoPtr & task_info, std::lock_guard<std::mutex> & task_schedule_mutex_lock);
    
    const size_t size;
    std::atomic<bool> shutdown {false};
    Threads threads;
    Poco::NotificationQueue queue;
    
    std::condition_variable wakeup_cond;
    std::mutex delayed_tasks_mutex;
    
    ThreadFromGlobalPool delayed_thread;
    
    DelayedTasks delayed_tasks;
    
    ThreadGroupStatusPtr thread_group;
    CurrentMetrics::Metric tasks_metric;
    CurrentMetrics::Metric memory_metric;
    std::string thread_name;
    void attachToThreadGroup();
};
class BackgroundSchedulePoolTaskInfo : public std::enable_shared_from_this<BackgroundSchedulePoolTaskInfo>, private boost::noncopyable