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.
#include <Common/formatReadable.h>
#include <Common/thread_local_rng.h>
#include <Compression/ICompressionCodec.h>
#include <Core/BackgroundSchedulePool.h>
#include <Formats/FormatFactory.h>
#include <Databases/IDatabase.h>
#include <Storages/IStorage.h>
#include <Storages/MarkCache.h>
#include <Storages/MergeTree/BackgroundProcessingPool.h>
#include <Storages/MergeTree/MergeList.h>
#include <Storages/MergeTree/MergeTreeSettings.h>
#include <Storages/CompressionCodecSelector.h>
#include <Storages/StorageS3Settings.h>
#include <Storages/LiveView/TemporaryLiveViewCleaner.h>
#include <Disks/DiskLocal.h>
#include <TableFunctions/TableFunctionFactory.h>
#include <Interpreters/ActionLocksManager.h>
#include <Core/Settings.h>
#include <Access/AccessControlManager.h>
#include <Access/ContextAccess.h>
#include <Access/EnabledRolesInfo.h>
#include <Access/EnabledRowPolicies.h>
#include <Access/QuotaUsage.h>
#include <Access/User.h>
#include <Access/SettingsProfile.h>
#include <Access/SettingsConstraints.h>
#include <Interpreters/ExpressionJIT.h>
#include <Dictionaries/Embedded/GeoDictionariesLoader.h>
#include <Interpreters/EmbeddedDictionaries.h>
#include <Interpreters/ExternalDictionariesLoader.h>