EXISTS key [key ...]
Time complexity: O(N) where N is the number of keys to check.
The user should be aware that if the same existing key is mentioned in the arguments multiple times, it will be counted multiple times. So if
EXISTS somekey somekey will return 2.
Integer reply, specifically the number of keys that exist from those specified as arguments.
dragonfly> SET key1 "Hello"
dragonfly> EXISTS key1
dragonfly> EXISTS nosuchkey
dragonfly> SET key2 "World"
dragonfly> EXISTS key1 key2 nosuchkey