Fix counter and gauge serialization bugs

* adjust `RollupRunnable` test to build preag metrics from single values (not rollups).
* and store them correctly.
* choose the right serializer for full-res data in `AR.getDataToRoll()`.
* add some comments about some refactoring I'd like to see.
* Adjust counter to handle all number types, not just longs (promote to double whenever encountered)
* Add raw computes for counter and gauges.