By Stephanie Merry, The Washington Post “Transformers: The Last Knight” begins in the Dark Ages, as King Arthur and Lancelot are preparing to enter a brutal battle. They try to stall, waiting for Merlin to materialize with some magical surprise. (Hint: It’s massive and made of metal.) “This is what the end looks like,” says a peasant, amid the carnage. If only. The fifth “Transformers” installment is just getting started, and already it’s exhausting.