Figure out why stream bandwidth is below 11MB for all relays

Working on #40190 (moved) @gk found that measuring nth relays with https://gitlab.torproject.org/juga/relay_bw, none of the chosen paths nor Web servers measure more than 11MB. Querying the new metrics DB, i realized that this is also the case for sbws.

Could this be due to some tor configuration or limitation that we haven't realized?

Working on tor_fusion#5 with hiro, i realized that https://gitlab.torproject.org/tpo/network-health/metrics/onionperf/-/blob/74074b0c61c68edf2c5a9f20d437ccf3d9c5125a/onionperf/visualization.py#L185 is calculating the throughput by taking only the time it takes to download the last MiB of data. I observed in the past sbws download speed was quite irregular, not that it was getting faster over time, but i might be wrong.

We didn't finish implementing onbasca#128 to check the bandwidth after the slow start.