ui: Change LiveBytes graph on Replicas page to reflect metric used in rebalancing

[a-robinson]

This is a follow-up issue to #17733 (comment). We changed rebalancing to balance on each store's TotalBytes, but there isn't a metric handy for that to graph. If we can graph the sum of two metrics (KeyBytes and ValBytes), we should do that, otherwise we'll need to expose a new metric for TotalBytes.

cc @mrtracy for whether we're capable of graphing the sum of two metrics.