inputs.internet_speed: upload speed severely under-reported in v1.40.0

Relevant telegraf.conf

# Test internet speed
[[inputs.internet_speed]]
  interval = "1h"
  collection_offset = "45m"
  memory_saving_mode = false
  test_mode = "multi"

Logs from Telegraf

No errors or warnings are logged, even with --debug. The plugin gathers and writes metrics successfully; only the value of the `upload` field is wrong.

System info

Telegraf 1.40.0, Docker 29.8.0, Debian 13 Trixie

Docker

No response

Steps to reproduce

1. Run [[inputs.internet_speed]] on a host whose real upload is well above the reported value (e.g. a ~100+ Mbps connection).
2. Record the upload field on Telegraf 1.39.x.
3. Upgrade to Telegraf 1.40.0 and record the upload field again for the same host/link.
4. Compare: download is unchanged and correct, but upload drops substantially.

Observed here: real upload >100 Mbps, reported upload ~28 Mbps; download reported ~118 Mbps and correct.

Expected behavior

The upload field should reflect actual upload throughput, consistent with pre-1.40.0 results and with browser/CLI speedtest runs from the same host.

Actual behavior

The upload field is significantly under-reported. The download field is unaffected.

Additional info

v1.40.0 bumped github.com/showwin/speedtest-go from 1.7.10 to 1.8.2 (via #19425 and #19596). The upload measurement changed in speedtest-go PR showwin/speedtest-go#267 ("fix(upload): count only confirmed request bytes").