I already read that and it doesn’t completely. It isn’t a big deal, because it resets after a minute or so, but I would really like to understand the limit properly. 
The reason for this is that while testing I noticed it wasn’t always used, only when I request the metric billed_charge_local_micro. As soon as I remove this metric I may request whatever I want even though it hasn’t been reset yet (X-Cost-Rate-Limit-Reset in the future and a X-Cost-Rate-Limit-Remaining of 0). This made me almost think it’s literally “cost” as in “spend metrics”, but the documentation doesn’t say anything about that…