Track gauge values and support default tags for dogstatsd

[jfhamlin]

Dogstatsd doesn't support deltas for gauges, so we must maintain the raw gauge value ourselves. lv.Spaces don't support carrying over values from previous flush periods, so this implementation duplicates lv.Space's label value tree logic to map to Gauges rather than observations. The simpler implementation of https://github.com/go-kit/kit/blob/master/metrics/graphite/graphite.go#L69 won't fly here because Dogstatsd supports tags, and we need to return the same Gauge every time for a given (name, lvs...) tuple. The changes also add tracking for whether a gauge was written in a flush period to avoid sending every gauge value every time.

Also adds support for the tag analogue of the prefix parameter to dogstatsd.New.

#578