bf: fix metrics reporting of request size
bf: fix metrics reporting of request size

ZENKOIO-101
This fixes a bug where Cloudserver will reports the byte size for a PUT request as NewSizeLength - OldSizeLength causing inaccuracies when putting Objects into a non versioned bucket. (e.g. I put a 100MB file into a bucket, the counter will reflect accurately, but if I put the same object again, the counter won't change even though 100MB just passed through).

scality/S3,scality/S3,scality/S3,scality/S3,scality/S3,scality/S3,scality/S3,scality/S3