Fix the meter type for LB Bytes

This fixes the type on bytes in/out to cumulative.
Eventually when connections are fixed in LBaaS side,
those would be cumulative too. Also update the
measurement docs to reflect the newest changes.

Closes-Bug: 1328573

Change-Id: Iab221829702ab36a844d0ae65c6a3312a311a92b
Merge "Fix the meter type for LB Bytes"