Retrieve the calling credential's rate limit state

What this credential has left, on every budget that applies to it, measured after this request has been counted.

Three budgets exist. general counts every request. audience counts only the operations that return subscriber or recipient email addresses. write counts only the operations that change something. The two narrow budgets are smaller than general, and the operations they cover are charged to both and have to pass both, so a credential that has exhausted one of them can still call everything else.

Read this rather than inferring the whole picture from the RateLimit-* headers, which describe one budget at a time.

This operation is itself counted against the general budget.