Return rate limit calculations as headers

It would be great if your ACME API endpoints returned headers that indicate rate limit values, similar to how GitHub does the same thing:

X-RateLimit-Limit: 60
X-RateLimit-Remaining: 56
X-RateLimit-Reset: 1372700873

crt.sh and CTLs are never going to be timely enough for use, and relying on customers to calculate something that you already have values for is also prone to mistakes. Why not just tell us the limits we’re under?

Another bonus here is that we don’t have to centrally manage counters in our distributed systems to see how close we are to a given rate limit – we can just look at our transactions.

ACME uses Retry-After for this. It’s been requested: https://github.com/letsencrypt/boulder/issues/4104

However, it’s not quite the same as what you’re asking for.

I think there’s a bit of a problem conceptually because some of the rate limits are an aggregation over the the rate limits for each FQDN. How you would represent that in response headers is beyond me - every certificate order would come up completely different numbers and I’m not sure how useful it’d be.

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I think a response that applies to the specific request would be adequate… e.g. if you request on these domains, you get back the rate limit for the specific request on those.

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