**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'
```
* - data insertion with a possible attach from local data (ATTACH),
  * - merge (MERGE),
  * - delete the partition (DROP).
  *
  * Each replica copies (queueUpdatingTask, pullLogsToQueue) entries from the log to its queue (/replicas/replica_name/queue/queue-...)
  *  and then executes them (queueTask).
  * Despite the name of the "queue", execution can be reordered, if necessary (shouldExecuteLogEntry, executeLogEntry).
  * In addition, the records in the queue can be generated independently (not from the log), in the following cases:
  * - when creating a new replica, actions are put on GET from other replicas (createReplica);
  * - if the part is corrupt (removePartAndEnqueueFetch) or absent during the check
  *   (at start - checkParts, while running - searchForMissingPart), actions are put on GET from other replicas;
  *
  * The replica to which INSERT was made in the queue will also have an entry of the GET of this data.
  * Such an entry is considered to be executed as soon as the queue handler sees it.
  *
  * The log entry has a creation time. This time is generated by the clock of server that created entry
  * - the one on which the corresponding INSERT or ALTER query came.
  *
  * For the entries in the queue that the replica made for itself,
  * as the time will take the time of creation the appropriate part on any of the replicas.
  */
class StorageReplicatedMergeTree final : public shared_ptr_helper<StorageReplicatedMergeTree>, public MergeTreeData
{
    friend struct shared_ptr_helper<StorageReplicatedMergeTree>;
public:
    void startup() override;
    void shutdown() override;
    ~StorageReplicatedMergeTree() override;
    std::string getName() const override { return "Replicated" + merging_params.getModeName() + "MergeTree"; }
    bool supportsParallelInsert() const override { return true; }