Switch website throughput metrics to use the last 1MB of the 5MB transfer
In onionperf#40020 (closed), @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.