When computing Measurements amari.kpi derives period from stats messages
- for example 60s if `xamari xlog` was running with stats/60s. But
inside that period there could be multiple x.drb_stats messages - for
example if xlog was running with `stats/60s x.drb_stats/10s`.
However the code was handling only the last such x.drb_stats instead of
accumulating counters from there during the period.

As the result, if. e.g. during the period there were 90B/9s and 2B/0.2s
x.drb_stats events, only the latter was accounted giving wrong data for
transmitted amount and transmission time.

-> Fix it.