By Zack Quaintance — The musical metaphors are too easy with DC Comics’ big summer event, Dark Nights: Death Metal. You can call it an encore to the first event — Dark Nights: Metal — a simple echo of ideas, inflated to new volume to give the audience more of the things they’ve cheered all along. Or, perhaps you can call it a sophomore album, a tricky followup to an earlier success that needs to check similar boxes while also going in new, stimulating directions.