Allow role without bracket on /benchmarks #2985

Split out of #2984 so it doesn't ride along with the approved change. The role param there only works on top of a bracket, and ?role=core alone returns 400, because we only write the bracket x role sets.

It would be more useful standalone. Someone comparing a hero's core and support distributions across all ranks has no way to ask for that today, and a param that only works in combination with another one is awkward to document. That would mean 2 more sets per hero and metric (benchmarks:{epoch}:{metric}:{heroId}:::core), against the 16 that bracket x role already adds, so it's a small fraction of what's there.

Not sure it's worth the keyspace, which is why it's a separate issue rather than something I slipped into the PR. Happy to leave it if you'd rather keep things tight.