In bf96c767 (kpi: Add way to compute aggregated counters + showcase
this) I added Calc.sum with the idea that aggregating measurements is
summing their values. That is indeed true for cumulative counters, but
to aggregate e.g. average values of something, one needs to use
different formula than simple summation. For this reason it is generally
more correct to name the aggregation method as "aggregate" instead of
"sum".

-> Do this renaming.