dvMiddlewareKeysWrapping top-level constant
Keys that wrap the handler rather than joining the chain.
dvTraced takes a handler and returns a response, so it is not a middleware in this chain's sense -- and it has to be outside the chain anyway. A request refused by a rate limit is still a request, and a trace that covers only the ones that got through is a latency graph with the slow half missing.
Implementation
const Set<String> dvMiddlewareKeysWrapping = <String>{'tracing'};