**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'
```
std::optional<String> replica_to_execute_merge;
    bool replica_to_execute_merge_picked = false;
    if (merge_strategy_picker.shouldMergeOnSingleReplica(entry))
    {
        replica_to_execute_merge = merge_strategy_picker.pickReplicaToExecuteMerge(entry);
        replica_to_execute_merge_picked = true;
        if (replica_to_execute_merge)
        {
            LOG_DEBUG(log,
                "Prefer fetching part {} from replica {} due to execute_merges_on_single_replica_time_threshold",
                entry.new_part_name, replica_to_execute_merge.value());
            return false;
        }
    }
    DataPartsVector parts;
    for (const String & source_part_name : entry.source_parts)
    {
        DataPartPtr source_part_or_covering = getActiveContainingPart(source_part_name);
        if (!source_part_or_covering)
        {
            
            LOG_DEBUG(log, "Don't have all parts for merge {}; will try to fetch it instead", entry.new_part_name);
            return false;
        }
        if (source_part_or_covering->name != source_part_name)
        {