expfmt: need to support gauge histogram metric type and float histogram

[beorn7]

The various MetricFamilyTo... functions are currently ignorant of the gauge histogram type recently added to the proto spec. Similarly, they ignore the float fields that got added to support a histogram where the counts are all floats.

Note that this is independent of the new native histograms. (The latter also support float histograms and gauge histograms, but so do the conventional histograms.)

[beorn7]

Even if we won't fully support those two new features, we at least have to handle them gracefully. Currently, we just error out with unknown metric type GAUGE_HISTOGRAM in the former case and silently ignore all the floats in the latter case.