B-48284 api-validator metrics instrumentation

Greg: Should the ACROSS ALL (via MeterByCategorySum) field track every invalid request as a single atomic unit, or every invalid request per role contained in that request? Right now, we are tracking every invalid role since it was more natural to implement. To track by request instead, we would likely need to use MeterByCategory instead, and only mark the ACROSS ALL field once for each invalid request rather than once for each role in an invalid request.