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.
{
    auto it = scalars.find(name);
    if (scalars.end() == it)
    {
        
        
        throw Exception("Scalar " + backQuoteIfNeed(name) + " doesn't exist (internal bug)", ErrorCodes::BAD_ARGUMENTS);
    }
    return it->second;
}
Tables Context::getExternalTables() const
{
    assert(global_context != this || getApplicationType() == ApplicationType::LOCAL);
    auto lock = getLock();
    Tables res;
    for (const auto & table : external_tables_mapping)
        res[table.first] = table.second->getTable();
    if (query_context && query_context != this)
    {
        Tables buf = query_context->getExternalTables();
        res.insert(buf.begin(), buf.end());
    }
    else if (session_context && session_context != this)
    {
        Tables buf = session_context->getExternalTables();
        res.insert(buf.begin(), buf.end());
    }
    return res;
}
void Context::addExternalTable(const String & table_name, TemporaryTableHolder && temporary_table)