Add support for gauge deltas.

* Add tests for gauges and new gauge deltas.
* Add support for +/- prefix for gauge values to support modifying
  gauge values in-place.

I realized after committing this that it makes it impossible to set an existing gauge to a negative number. An option would be to make a new "type" (e.g. `gd` for "gauge delta") that would modify existing gauge values in-place. Thoughts?