Add Summary Metric Type

Similar to a histogram, a summary samples observations (usually things like request durations and response sizes). While it also provides a total count of observations and a sum of all observed values, it calculates configurable quantiles over a sliding time window.

A summary with a base metric name of exposes multiple time series during a scrape:

streaming φ-quantiles (0 ≤ φ ≤ 1) of observed events, exposed as {quantile="<φ>"} the total sum of all observed values, exposed as _sum the count of events that have been observed, exposed as _count See histograms and summaries for detailed explanations of φ-quantiles, summary usage, and differences to histograms.

https://prometheus.io/docs/concepts/metric_types/#summary

Hi @LKaemmerling, is it a feature you'll be interested to include in this library ?

If yes, I would be glad to work on a PR inspired by the Java and Go implementations.

Hey @mp3000mp,

sure :) Feel free to work on a MR and i will look over it.