**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'
```
{
    if (fetcher.blocker.isCancelled())
    {
        disable_reason = fmt::format("Not executing fetch of part {} because replicated fetches are cancelled now.", entry.new_part_name);
        return false;
    }
    size_t busy_threads_in_pool = CurrentMetrics::values[CurrentMetrics::BackgroundFetchesPoolTask].load(std::memory_order_relaxed);
    if (busy_threads_in_pool >= replicated_fetches_pool_size)
    {
        disable_reason = fmt::format("Not executing fetch of part {} because {} fetches already executing, max {}.", entry.new_part_name, busy_threads_in_pool, replicated_fetches_pool_size);
        return false;
    }
    if (replicated_fetches_throttler->isThrottling())
    {
        disable_reason = fmt::format("Not executing fetch of part {} because fetches have already throttled by network settings "
                                     "<max_replicated_fetches_network_bandwidth> or <max_replicated_fetches_network_bandwidth_for_server>.", entry.new_part_name);
        return false;
    }
    return true;
}
bool StorageReplicatedMergeTree::partIsAssignedToBackgroundOperation(const DataPartPtr & part) const
{
    return queue.isVirtualPart(part);
}
void StorageReplicatedMergeTree::mergeSelectingTask()