Feature request: unenforced / test-only limits

Would be used as a precaution against being accidentally overly-aggressive when setting new rate limits on existing systems.

It'd probably be useful if both real and check-only limits could co-exist for the same rate limit pattern, to help with testing whether it would be safe to lower an existing rate limit.

I'm imagining something like `check_only_requests_per_unit` or `test_requests_per_unit` that could be set side-by-side with or in place of `requests_per_unit` and increment a different metric name instead of the existing over-limit metric (and near-limit metric) when it would have triggered.