
**Bug description**
Unlike `cluster()`, `clusterAllReplicas()` does not implement `_shard_num`. 

I would expect
```
SELECT _shard_num
FROM clusterAllReplicas(prod, system.one)
ORDER BY _shard_num ASC
FORMAT PrettyCompactMonoBlock

┌─_shard_num─┐
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
│          0 │
└────────────┘
```
to have an output more useful like
```
SELECT _shard_num
FROM cluster(prod, system.one)
ORDER BY _shard_num ASC
FORMAT PrettyCompactMonoBlock

┌─_shard_num─┐
│          1 │
│          2 │
│          3 │
│          4 │
│          5 │
│          6 │
│          7 │
│          8 │
│          9 │
│         10 │
│         11 │
│         12 │
│         13 │
│         14 │
│         15 │
└────────────┘
```

**Releases where this has been tested**

* v20.3.12.112-stable

**References**

[cluster, clusterAllReplicas documentation](https://clickhouse.tech/docs/en/sql-reference/table-functions/cluster/)

[_shard_num documentation](https://clickhouse.tech/docs/en/engines/table-engines/special/distributed/#virtual-columns)

{
    return std::unique_ptr<Cluster>{ new Cluster(ReplicasAsShardsTag{}, *this, settings)};
}
std::unique_ptr<Cluster> Cluster::getClusterWithSingleShard(size_t index) const
{
    return std::unique_ptr<Cluster>{ new Cluster(SubclusterTag{}, *this, {index}) };
}
std::unique_ptr<Cluster> Cluster::getClusterWithMultipleShards(const std::vector<size_t> & indices) const
{
    return std::unique_ptr<Cluster>{ new Cluster(SubclusterTag{}, *this, indices) };
}
Cluster::Cluster(Cluster::ReplicasAsShardsTag, const Cluster & from, const Settings & settings)
{
    if (from.addresses_with_failover.empty())
        throw Exception("Cluster is empty", ErrorCodes::LOGICAL_ERROR);
    std::set<std::pair<String, int>> unique_hosts;
    for (size_t shard_index : ext::range(0, from.shards_info.size()))
    {