**Use case**

Currently, `system.replicas` table has `total_replicas` and `active_replicas` columns which are useful to check/alert when some replicas are down. One problem, these columns are numbers. They tell when there is a problem, but you need to further check ZooKeeper or query all the nodes to detect what exact replicas (hosts) are down/inactive/have problems.

**Describe the solution you'd like**

Add a replicas column maybe as a map which would contain the replica name, and it's status. Or, add 2 array columns with `replica_names` and `active_replica_names`

**Alternative**

Something along the line

```sql
select path, value from system.zookeeper where path IN (
    select concat(path, '/', name) from system.zookeeper where path in (select concat(zookeeper_path, '/replicas') from system.replicas)
)
and name = 'is_active'
```
{
    const String & source_part_name = entry.source_parts.at(0);
    const auto storage_settings_ptr = getSettings();
    LOG_TRACE(log, "Executing log entry to mutate part {} to {}", source_part_name, entry.new_part_name);
    DataPartPtr source_part = getActiveContainingPart(source_part_name);
    if (!source_part)
    {
        LOG_DEBUG(log, "Source part {} for {} is not ready; will try to fetch it instead", source_part_name, entry.new_part_name);
        return false;
    }
    if (source_part->name != source_part_name)
    {
        LOG_WARNING(log, "Part " + source_part_name + " is covered by " + source_part->name
                    + " but should be mutated to " + entry.new_part_name + ". "
                    + "Possibly the mutation of this part is not needed and will be skipped. This shouldn't happen often.");
        return false;
    }
    
    size_t estimated_space_for_result = MergeTreeDataMergerMutator::estimateNeededDiskSpace({source_part});
    if (entry.create_time + storage_settings_ptr->prefer_fetch_merged_part_time_threshold.totalSeconds() <= time(nullptr)
        && estimated_space_for_result >= storage_settings_ptr->prefer_fetch_merged_part_size_threshold)
    {