**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'
```
future_merged_part.updatePath(*this, reserved_space);
    future_merged_part.merge_type = entry.merge_type;
    if (reserved_space->getDisk()->supportZeroCopyReplication()
        && storage_settings_ptr->allow_remote_fs_zero_copy_replication
        && merge_strategy_picker.shouldMergeOnSingleReplicaShared(entry))
    {
        if (!replica_to_execute_merge_picked)
            replica_to_execute_merge = merge_strategy_picker.pickReplicaToExecuteMerge(entry);
        if (replica_to_execute_merge)
        {
            LOG_DEBUG(log,
                "Prefer fetching part {} from replica {} due to remote_fs_execute_merges_on_single_replica_time_threshold",
                entry.new_part_name, replica_to_execute_merge.value());
            return false;
        }
    }
    
    if (isTTLMergeType(future_merged_part.merge_type))
        getContext()->getMergeList().bookMergeWithTTL();
    auto table_id = getStorageID();
    
    MergeList::EntryPtr merge_entry = getContext()->getMergeList().insert(getStorageID(), future_merged_part);
    Transaction transaction(*this);
    MutableDataPartPtr part;
    Stopwatch stopwatch;
    auto write_part_log = [&] (const ExecutionStatus & execution_status)
    {
        writePartLog(
            PartLogElement::MERGE_PARTS, execution_status, stopwatch.elapsed(),
            entry.new_part_name, part, parts, merge_entry.get());