Proposal: Add WithinLimit Counter to RateLimitStats

Hi team, I'm wondering if we can add a counter that counts the number of requests that is within limit (not over limit).

The reason is that we need to keep track how many requests are allowed by the rate limit service per second. We have some rate limits that don't allow any traffic and we want to monitor and make sure that no requests are going through. We are currently calculating it using
`rate(total_hits) - rate(over_limit)`

So for rate limits with 0 as the limit, this query should always return 0; however, prometheus' (we export the stastd metrics to prometheus) rate function does some smoothing/extrapolation and we often get non-zero numbers for those rate limits.

If we can add the WithinLimit counter, we can use that directly for monitoring allowed per second. Any help is appreciated. Thanks!