In act 3, scene 3 of Romeo and Juliet, during Romeo's monologue "'Tis torture, and not mercy; heaven is here," what figurative language and emotion does Shakespeare use?

Romeo and Juliet is a play that is, at its core, centered around emotional turbulence, focused as it is around themes of romantic intensity and vendetta. It's violent and tempestuous, and that emotional turbulence is reflected in the character of Romeo himself. Romeo's emotional intensity is demonstrated throughout the play, and indeed, that same emotional intensity is present in this scene as well.
Romeo's dominant mood at this point of the play is despair. After killing Tybalt, he has just received news that he has been punished with banishment, which will separate him from Juliet. For Romeo, death and separation amount to much the same thing, regardless of how the Friar might try to argue otherwise. This is the main emotional pull of the monologue in question: learning of his banishment, Romeo is pushed into despair, holding that life for him is effectively over. (Additionally, it might be worth noting that he does not yet know Juliet's reaction to the news of Tybalt's death, or his own part in it, a theme which will feature later in that same scene.)
Again, do keep in mind that these lines are written within the context of a conversation and represent Romeo's response to Friar Laurence's attempts at consolation. Laurence tells him that in banishing Romeo, the Prince is showing mercy, to which Romeo responds, as you quote, that this is "torture, not mercy." Indeed, there is a lengthy metaphor that runs across these lines, by which the traditional Christian poles between mortality and salvation are switched, so that heaven becomes present within life rather than in the afterlife, because, for Romeo, heaven is to be in the presence of Juliet herself. Thus, he states that

every cat, and dogAnd little mouse, every worthy thing,Live here in heaven and may look on her;But Romeo may not.—More validity,More honourable state, more courtship livesIn carrion flies than Romeo

We can see here also an element of self-pity, with the way Romeo casts himself as the most sorrowful creature on earth and through the unfavorable comparisons he draws. It is his contention that, by being banished, he might as well be dead.