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.
SharedContextHolder::~SharedContextHolder() = default;
SharedContextHolder::SharedContextHolder(std::unique_ptr<ContextShared> shared_context)
    : shared(std::move(shared_context)) {}
void SharedContextHolder::reset() { shared.reset(); }
Context Context::createGlobal(ContextShared * shared)
{
    Context res;
    res.shared = shared;
    return res;
}
void Context::initGlobal()
{
    DatabaseCatalog::init(this);
    TemporaryLiveViewCleaner::init(*this);
}
SharedContextHolder Context::createShared()
{
    return SharedContextHolder(std::make_unique<ContextShared>());
}
Context::~Context() = default;
InterserverIOHandler & Context::getInterserverIOHandler() { return shared->interserver_io_handler; }
std::unique_lock<std::recursive_mutex> Context::getLock() const
{
    ProfileEvents::increment(ProfileEvents::ContextLock);
    CurrentMetrics::Increment increment{CurrentMetrics::ContextLockWait};
    return std::unique_lock(shared->mutex);
}
ProcessList & Context::getProcessList() { return shared->process_list; }
const ProcessList & Context::getProcessList() const { return shared->process_list; }