HKEYS
Syntax
        HKEYS key
- Available since:
- 2.0.0
- Time complexity:
- O(N) where N is the size of the hash.
- ACL categories:
- 
              
                @read,@hash,@slow,
Returns all field names in the hash stored at key.
Return
Array reply: list of fields in the hash, or an empty list when key does
not exist.