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.
extern const Metric BackgroundMovePoolTask;
    extern const Metric MemoryTrackingInBackgroundMoveProcessingPool;
    extern const Metric BackgroundSchedulePoolTask;
    extern const Metric MemoryTrackingInBackgroundSchedulePool;
    extern const Metric BackgroundBufferFlushSchedulePoolTask;
    extern const Metric MemoryTrackingInBackgroundBufferFlushSchedulePool;
    extern const Metric BackgroundDistributedSchedulePoolTask;
    extern const Metric MemoryTrackingInBackgroundDistributedSchedulePool;
    extern const Metric BackgroundMessageBrokerSchedulePoolTask;
    extern const Metric MemoryTrackingInBackgroundMessageBrokerSchedulePool;
}
namespace DB
{
namespace ErrorCodes
{
    extern const int BAD_ARGUMENTS;
    extern const int BAD_GET;
    extern const int UNKNOWN_DATABASE;
    extern const int UNKNOWN_TABLE;
    extern const int TABLE_ALREADY_EXISTS;
    extern const int THERE_IS_NO_SESSION;
    extern const int THERE_IS_NO_QUERY;
    extern const int NO_ELEMENTS_IN_CONFIG;
    extern const int TABLE_SIZE_EXCEEDS_MAX_DROP_SIZE_LIMIT;
    extern const int SESSION_NOT_FOUND;
    extern const int SESSION_IS_LOCKED;
    extern const int LOGICAL_ERROR;
    extern const int NOT_IMPLEMENTED;
}
class NamedSessions