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.
MergeList & Context::getMergeList() { return shared->merge_list; }
const MergeList & Context::getMergeList() const { return shared->merge_list; }
void Context::enableNamedSessions()
{
    shared->named_sessions.emplace();
}
std::shared_ptr<NamedSession> Context::acquireNamedSession(const String & session_id, std::chrono::steady_clock::duration timeout, bool session_check)
{
    if (!shared->named_sessions)
        throw Exception("Support for named sessions is not enabled", ErrorCodes::NOT_IMPLEMENTED);
    return shared->named_sessions->acquireSession(session_id, *this, timeout, session_check);
}
String Context::resolveDatabase(const String & database_name) const
{
    String res = database_name.empty() ? getCurrentDatabase() : database_name;
    if (res.empty())
        throw Exception("Default database is not selected", ErrorCodes::UNKNOWN_DATABASE);
    return res;
}
String Context::getPath() const
{
    auto lock = getLock();
    return shared->path;
}
String Context::getFlagsPath() const
{
    auto lock = getLock();
    return shared->flags_path;
}
String Context::getUserFilesPath() const