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.
class BackgroundSchedulePoolTaskInfo;
class BackgroundSchedulePoolTaskHolder;
/** Executes functions scheduled at a specific point in time.
  * Basically all tasks are added in a queue and precessed by worker threads.
  *
  * The most important difference between this and BackgroundProcessingPool
  *  is that we have the guarantee that the same function is not executed from many workers in the same time.
  *
  * The usage scenario: instead starting a separate thread for each task,
  *  register a task in BackgroundSchedulePool and when you need to run the task,
  *  call schedule or scheduleAfter(duration) method.
  */
class BackgroundSchedulePool
{
public:
    friend class BackgroundSchedulePoolTaskInfo;
    using TaskInfo = BackgroundSchedulePoolTaskInfo;
    using TaskInfoPtr = std::shared_ptr<TaskInfo>;
    using TaskFunc = std::function<void()>;
    using TaskHolder = BackgroundSchedulePoolTaskHolder;
    using DelayedTasks = std::multimap<Poco::Timestamp, TaskInfoPtr>;
    TaskHolder createTask(const std::string & log_name, const TaskFunc & function);
    size_t getNumberOfThreads() const { return size; }