Specify data format and aggregation process of statistics offered by metrics.tp.o

As soon as we know what graphs we should make for #23761 (moved), we'll have to 1) specify the data format of the CSV file we need to produce for that.

And for Sponsor 13 we'll need to 2) specify the aggregation process, which will enable others to reproduce our results.

I'll start with 1) as soon as we have green light from #23761 (moved), but I'd appreciate help with 2).