**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 IMergeTreeDataPart & part, DiskType::Type disk_type) const
{
    String best_replica;
    zkutil::ZooKeeperPtr zookeeper = tryGetZooKeeper();
    if (!zookeeper)
        return best_replica;
    String zero_copy = fmt::format("zero_copy_{}", DiskType::toString(disk_type));
    String zookeeper_part_node = fs::path(zookeeper_path) / zero_copy / "shared" / part.name;
    Strings ids;
    zookeeper->tryGetChildren(zookeeper_part_node, ids);
    Strings replicas;
    for (const auto & id : ids)
    {
        String zookeeper_part_uniq_node = fs::path(zookeeper_part_node) / id;
        Strings id_replicas;
        zookeeper->tryGetChildren(zookeeper_part_uniq_node, id_replicas);
        LOG_TRACE(log, "Found zookeper replicas for {}: {}", zookeeper_part_uniq_node, id_replicas.size());
        replicas.insert(replicas.end(), id_replicas.begin(), id_replicas.end());
    }
    LOG_TRACE(log, "Found zookeper replicas for part {}: {}", part.name, replicas.size());
    Strings active_replicas;
    
    
    active_replicas.reserve(replicas.size());
    for (const String & replica : replicas)
        if ((replica != replica_name) && (zookeeper->exists(fs::path(zookeeper_path) / "replicas" / replica / "is_active")))
            active_replicas.push_back(replica);