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.
const PoolSettings & pool_settings,
        const char * log_name,
        const char * thread_name_)
    : size(size_)
    , thread_name(thread_name_)
    , settings(pool_settings)
{
    logger = &Poco::Logger::get(log_name);
    LOG_INFO(logger, "Create {} with {} threads", log_name, size);
    threads.resize(size);
    for (auto & thread : threads)
        thread = ThreadFromGlobalPool([this] { workLoopFunc(); });
}
BackgroundProcessingPool::TaskHandle BackgroundProcessingPool::createTask(const Task & task)
{
    return std::make_shared<TaskInfo>(*this, task);
}
void BackgroundProcessingPool::startTask(const TaskHandle & task, bool allow_execute_in_parallel)