Returns the number of entries inside a stream. If the specified key does not
exist the command returns zero, as if the stream was empty. However note that
unlike other Redis types, zero-length streams are possible, so you should call
`TYPE` or `EXISTS` in order to check if a key exists or not.

Streams are not auto-deleted once they have no entries inside (for instance
after an `XDEL` call), because the stream may have consumer groups associated
with it.

@return

@integer-reply: the number of entries of the stream at `key`.

@examples

```cli
XADD mystream * item 1
XADD mystream * item 2
XADD mystream * item 3
XLEN mystream
```