Retrieving current API usage levels and RETRY-AFTER

Is there an API call that can be made to find out how many requests have been made so it can be compared against the daily quota? It would be great to be able to query that. We have a client that gets a 429 but the claim is that they aren't making too many requests. Getting statistics by making an API call would be helpful to manage the account. For example, seeing that they have used 4,000 of the 10,000 daily limit would be helpful.

In addition, when a 429 is received, it would be good to get a "RETRY-AFTER" header. so that we could pause any operations until that time.

I have looked for extra headers when a 429 error occurs but don't see any that might provide this information.

TIA