Add transferred snapshot data size to logs output

[MurzNN]

At now logs output are shows only duration of transfer process like this:

INFO backup sx-20210830-070013-utc created successfully in 00:00:10

Will be good to have here also information about amount of transferred data. It can be visible in pv output like this:

88.6KiB 0:00:00 [ 289KiB/s] [ <=>                                                                      ]

But for catch the number in scripts will be much better to have a separate formatted log line with this number.

[MurzNN]

I investigated that transfer size can be calculated and dropped to output using pv -bn command, so we can reuse it to calculate transferred size inside pipe.