Make metric.Gauge a float64

[nightlyone]

Rationale: Gauges are measured in various accuracies and are difficult to rescale later once you already have collected data. The performance impact seems negligible, since we only Set and Add Gauges. Doubling the memory consumption per Gauge seems worth the result.

[peterbourgon]

Originally Gauge was float64, but I changed to int64 after reviewing codahale/metrics. I like the integer consistency of the metrics interfaces, especially as e.g. Histograms are implemented in terms of Gauges in some places. And I can't think of many (any?) things that are Gauge-able that can't be reduced to integers. Can you link me to some code in the wild that requires Gauges-as-floats?

[peterbourgon]

Closed pending further comments...