Grice does not deal with similes, but with metaphors directly.
You, I tell you, are the cream in my coffee.
This is of course TOTALLY different from
You, I tell you, are LIKE the cream in my coffee.
But since the former is a categorial mistake and obviously a FALSE thing to say, it's best to see the logical form of metaphor as involving a 'prompt of comparison'.
"Shakespeare used a lot of these prompts," but then he was an actor!