Observability: Track bytes per transaction

[joshua-spacetime]

Motivation: Reducer timing spans do not account for all work done in the transaction. In particular they do not take into account the final step of updating the committed state. Tracking bytes modified/written will provide more clarity when there is a discrepancy between timing spans and reducer duration metrics.

[cloutiertyler]

The energy accounting may need a similar metric, which will be calculated based on work in another ticket.