**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'
```
#include <Interpreters/PartLog.h>
#include <Common/randomSeed.h>
#include <Common/ZooKeeper/ZooKeeper.h>
#include <Common/Throttler.h>
#include <Core/BackgroundSchedulePool.h>
#include <Processors/Pipe.h>
#include <Storages/MergeTree/BackgroundJobsExecutor.h>
namespace DB
{
/** The engine that uses the merge tree (see MergeTreeData) and is replicated through ZooKeeper.
  *
  * ZooKeeper is used for the following things:
  * - the structure of the table (/metadata, /columns)
  * - action log with data (/log/log-...,/replicas/replica_name/queue/queue-...);
  * - a replica list (/replicas), and replica activity tag (/replicas/replica_name/is_active), replica addresses (/replicas/replica_name/host);
  * - the leader replica election (/leader_election) - these are the replicas that assign merges, mutations
  *   and partition manipulations.
  *   (after ClickHouse version 20.5 we allow multiple leaders to act concurrently);
  * - a set of parts of data on each replica (/replicas/replica_name/parts);
  * - list of the last N blocks of data with checksum, for deduplication (/blocks);
  * - the list of incremental block numbers (/block_numbers) that we are about to insert,
  *   to ensure the linear order of data insertion and data merge only on the intervals in this sequence;
  * - coordinate writes with quorum (/quorum).
  * - Storage of mutation entries (ALTER DELETE, ALTER UPDATE etc.) to execute (/mutations).
  *   See comments in StorageReplicatedMergeTree::mutate() for details.
  */
/** The replicated tables have a common log (/log/log-...).
  * Log - a sequence of entries (LogEntry) about what to do.
  * Each entry is one of:
  * - normal data insertion (GET),