Reading the Tyk documentation, it seems the only metric for rate limiting, quotas, and billing is the number of API calls. We'd like to be able to rate limit and bill based on data transfer amount, e.g. the total Megabytes transferred into our API in the past month, by a particular application (or the total data transfer out).
Is that actually possible today? If so, is it documented anywhere? If not, how hard would it be to do?