Add `data` metrics to measure payload rate.
Rename `total` metrics to `success`, because these metrics count
success operations count but not total operations count.
Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>
Return an interface which can be converted to `io.Reader` as well as
report payload size and hash.
Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>
