How to log the actual transfer size reported by btrbk?

> Partly related to #280

I try to capture all logs in order to parse later to get statistics, like so:

btrbk -c conf.calculated --progress -v run | tee mylog.txt

However, the transfer size information is not present in the `mylog.txt`. How can I redirect the `summary: 50.7 MiByte in 4.0sec - average of 12.6 MiB/s` line into the log file?

Start by reproducing the shown btrbk command with --progress and -v, capturing output in mylog.txt, and determine where the summary line is written. Check the CLI logging and output behavior, then document the supported way to capture the transfer summary so the result can be verified from the log.

Written by the indexing model from the issue text.