Summary metric lacks a decay window

[mithrandi]

I might be misunderstanding, but the Prometheus documentation talks about a sliding window or decay period for summary metrics; however the current implementation seems to aggregate over all time (since the metric was initialized), which seems to be less useful.

[mithrandi]

I see the Python client library doesn't emit implement the quantile metrics at all (a summary only has the count and sum metrics), deliberately; perhaps we should think about deprecating these.