**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'
```
auto current_metadata = getInMemoryMetadataPtr();
        StorageInMemoryMetadata future_metadata = *current_metadata;
        commands.apply(future_metadata, query_context);
        ReplicatedMergeTreeTableMetadata future_metadata_in_zk(*this, current_metadata);
        if (ast_to_str(future_metadata.sorting_key.definition_ast) != ast_to_str(current_metadata->sorting_key.definition_ast))
        {
            
            
            
            
            future_metadata_in_zk.sorting_key = serializeAST(*extractKeyExpressionList(future_metadata.sorting_key.definition_ast));
        }
        if (ast_to_str(future_metadata.sampling_key.definition_ast) != ast_to_str(current_metadata->sampling_key.definition_ast))
            future_metadata_in_zk.sampling_expression = serializeAST(*extractKeyExpressionList(future_metadata.sampling_key.definition_ast));
        if (ast_to_str(future_metadata.partition_key.definition_ast) != ast_to_str(current_metadata->partition_key.definition_ast))
            future_metadata_in_zk.partition_key = serializeAST(*extractKeyExpressionList(future_metadata.partition_key.definition_ast));
        if (ast_to_str(future_metadata.table_ttl.definition_ast) != ast_to_str(current_metadata->table_ttl.definition_ast))
        {
            if (future_metadata.table_ttl.definition_ast)
                future_metadata_in_zk.ttl_table = serializeAST(*future_metadata.table_ttl.definition_ast);
            else 
                future_metadata_in_zk.ttl_table = "";
        }
        String new_indices_str = future_metadata.secondary_indices.toString();
        if (new_indices_str != current_metadata->secondary_indices.toString())
            future_metadata_in_zk.skip_indices = new_indices_str;
        String new_projections_str = future_metadata.projections.toString();
        if (new_projections_str != current_metadata->projections.toString())
            future_metadata_in_zk.projections = new_projections_str;
        String new_constraints_str = future_metadata.constraints.toString();