Don't aggregate non-numerical gauges.

Gauges in the metrics-core library can have any type, but when being aggregated in multimeter gauges are assumed to be numerical (or nil). This is an unneccessary contraint imo, and could be avoided by simply not aggregating non-numerical gauges.

On a side note, the aggregation has become more and more messy, and should probably be cleaned up whenever we move on to metrics-core 3.0.