Add option to expire internal metrics by tracking recency

[bruceg]

Recency provides the metric storage wrapper and logic to detect metrics that have truly not been updated within a given time period. This is required because gauges could technically be the same value when observed at two different times, even if it was in fact changed in between those two observations. Recency deals with this by tracking a "generation" for a metric, such that any operation on the metric at all increments an internal counter -- the generation -- so that you can tell, for both of those two hypothetical observations mentioned above: "oh, the generation is identical, so it hasn't been updated" or "oh, the generations are different, so it's definitely still being updated".