Relates tocoder/internal#889

The existing implementation for exposing read and written bytes was a little awkward - we're going to be adding a bunch of scaletest runners / load generators thatdon't transfer any bytes. This PR has the scaletest reports expose a map of arbitrary string-keyed metrics instead.

FWIW, the latest iteration of the scaletesting infrastructure doesn't parse these reports right now - they're just logged to stdout, so we're good to break the json schema here.