Add prometheus counter to firehose to know how often the `firehose-rate-limit-bucket` is hit

[matthewdarwin]

We have the following configured on firehose:

    firehose-rate-limit-bucket-size: 5
    firehose-rate-limit-bucket-fill-rate: 2s

It would be great to have a prometheus counter to show how often the rate limits are hit so we know when we need to adjust.
I looked through the existing metrics and didn't see anything that matches