Returns the number of keys in the specified Redis Cluster hash slot. The command
only queries the local data set, so contacting a node that is not serving the
specified hash slot will always result in a count of zero being returned.

```
> CLUSTER COUNTKEYSINSLOT 7000
(integer) 50341
```

@return

@integer-reply: The number of keys in the specified hash slot, or an error if
the hash slot is invalid.