Add units to all stats

We currently don't expose the units for each stat that we publish under `/_node//_stats`. We should.

I bet this is a pretty straightforward update to all of the `stats_descriptions.cfg` files for someone who knows what all of the stats are.

Start by locating all stats_descriptions.cfg files used for /_node//_stats and review the existing stat definitions. Determine the correct unit for every published stat, then verify that the endpoint exposes those units consistently.

Written by the indexing model from the issue text.