[twic]

Yes, exactly this. It's the fact that you're doing aggregation in two places. Since you're always going to be aggregating on the backend, aggregating in the app is bad news.

It may be interesting to think about the class of aggregate metrics that you can safely aggregate. Totals can be summed. Counts can be summed. Maxima can be maxed. Minima can be minned. Histograms can be summed (but histograms are lossy). A pair of aggregatable metrics can be aggregated pairwise; a pair of a total and a count lets you find an average.

Medians and quantiles, though, can't be combined, and those are what we want most of the time.

Someone who loves functional programming can tell us if metrics in this class are monoids or what.

There is an unjustly obscure beast called a t-digest which is a bit like an adaptive histogram; it provides a way to aggregate numbers such that you can extract medians and quantiles, and the aggregates can be combined:

https://github.com/tdunning/t-digest