Returns the string length of the value associated with `field` in the hash
stored at `key`. If the `key` or the `field` do not exist, 0 is returned.

@return

@integer-reply: the string length of the value associated with `field`, or zero
when `field` is not present in the hash or `key` does not exist at all.

@examples

```cli
HMSET myhash f1 HelloWorld f2 99 f3 -256
HSTRLEN myhash f1
HSTRLEN myhash f2
HSTRLEN myhash f3
```