Unfortunately, as far as I know, rates are not in the product plan. The data behind the performance graphs is available but that's not what you are going for. I'm not sure what the reason is for not exposing rates outside from the fact that they are not used internally. Really the only way to get rates is to do it on the client side. There are things you can do like check if the counter values are smaller than the last one you collected and if so assume either a rollover of the stats (2^64) occurred or a reset of the stats occurred. Then you can skip that polling cycle and start on the next one for your rates.
If this is important for you, then I'd suggest you get in touch with professional services and make a official request for this feature.
-Joe
Nimbostratus