**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'
```
MutationCommands getFirstAlterMutationCommandsForPart(const DataPartPtr & part) const override;
    void startBackgroundMovesIfNeeded() override;
    std::unique_ptr<MergeTreeSettings> getDefaultSettings() const override;
    std::set<String> getPartitionIdsAffectedByCommands(const MutationCommands & commands, ContextPtr query_context) const;
    PartitionBlockNumbersHolder allocateBlockNumbersInAffectedPartitions(
        const MutationCommands & commands, ContextPtr query_context, const zkutil::ZooKeeperPtr & zookeeper) const;
protected:
    /** If not 'attach', either creates a new table in ZK, or adds a replica to an existing table.
      */
    StorageReplicatedMergeTree(
        const String & zookeeper_path_,
        const String & replica_name_,
        bool attach,
        const StorageID & table_id_,
        const String & relative_data_path_,
        const StorageInMemoryMetadata & metadata_,
        ContextMutablePtr context_,
        const String & date_column_name,
        const MergingParams & merging_params_,
        std::unique_ptr<MergeTreeSettings> settings_,
        bool has_force_restore_data_flag,
        bool allow_renaming_);
};
/** There are three places for each part, where it should be
  * 1. In the RAM, data_parts, all_data_parts.
  * 2. In the filesystem (FS), the directory with the data of the table.
  * 3. in ZooKeeper (ZK).
  *
  * When adding a part, it must be added immediately to these three places.
  * This is done like this