'My vegetable love should grow / Vaster than empires, and more slow' - Which metaphor has Andrew Marvell used in these lines from his poem 'To His Coy Mistress'?

A metaphor states that one thing is something else. This description, however, is too simple for the way metaphors often work in poetry, literature and speeches. You will often find something being described, or written about, as if it is something else, without the writer ever saying 'x is y' (do you see the mathematical metaphor there?).