go/vt/mysqlctl/backupstats: increment *_bytes metric by actual number of bytes

[maxenglander]

Description

Recently merged detailed backup stats has a minor bug in that it is not actually counting # of bytes. I know I had this correct at least locally at some point, but looking through commits in #11979 it seems like it was always wrong 😵‍💫

Related Issue(s)

#11979

Checklist

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[deepthi]

Can you enhance one of the existing backup/restore endtoend tests to validate all the new stats?

[maxenglander]

@deepthi can do!

[deepthi]

LGTM