Switch website throughput metrics to use the last 1MB of the 5MB transfer

In https://gitlab.torproject.org/tpo/metrics/onionperf/-/issues/40020, @acute switched the onionperf graphs of throughput to use the last 1MB of a 5MB transfer. We should do this for the website, too. (Congestion control may need some time to arrive at steady-state throughput, so this is a better throughput metric).

I would tag this as S61-O1-Maybe, but gitlab won't let me. We will be looking at these graphs during live experiments, and giving them to Sponsor61 in our reports.