To control this, we use an environment variable. This is because
Rack::Attack is configured in an initialiser, so a feature flag is of
minimal value - it would still need a restart to take effect.

We could rewrite the InstrumentedCacheStore to allow changing the store
on a per-operation basis, but that adds more risk for what should be a
quick migration. It would also add a feature flag check in a very hot
code path (rate limiting checks happen multiple times on every request).