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.
, result_part_path{future_part.path}
    , result_data_version{future_part.part_info.getDataVersion()}
    , num_parts{future_part.parts.size()}
    , thread_id{getThreadId()}
    , merge_type{future_part.merge_type}
    , merge_algorithm{MergeAlgorithm::Undecided}
{
    for (const auto & source_part : future_part.parts)
    {
        source_part_names.emplace_back(source_part->name);
        source_part_paths.emplace_back(source_part->getFullPath());
        total_size_bytes_compressed += source_part->getBytesOnDisk();
        total_size_marks += source_part->getMarksCount();
        total_rows_count += source_part->index_granularity.getTotalRows();
    }
    if (!future_part.parts.empty())
    {
        source_data_version = future_part.parts[0]->info.getDataVersion();
        is_mutation = (result_data_version != source_data_version);
    }
    
    background_thread_memory_tracker = CurrentThread::getMemoryTracker();
    if (background_thread_memory_tracker)
    {
        memory_tracker.setMetric(CurrentMetrics::MemoryTrackingForMerges);
        background_thread_memory_tracker_prev_parent = background_thread_memory_tracker->getParent();
        background_thread_memory_tracker->setParent(&memory_tracker);
    }
}
MergeInfo MergeListElement::getInfo() const