Have you considered pushing the aggregation into the applications, rather than doing it across the network?

Having whatever library is sending data to statsd, instead keep counters/gauges in memory and then expose that on a regular basis would greatly reduce the data volumes involved as it's O(timeseries*frequency) rather than O(events).

This is the approach we take with Prometheus, and based on the statsd setups of some people who've come talking to us there's scope for a reduction in network load of at least an order of magnitude without having to do any downsampling.