Full summary implementation

[deadtrickster]

Now only basic variant of summary metric is implemented - with sum and count of observations.

What's left:

• [ ] Quantiles;
• [ ] Configurable sliding window

Prometheus summaries usually use highly biased quantile and since we deal with streaming data so they always implement the same algorithm from paper [0]. Example implementations: [1-3].

Sliding window seems to be implemented using N numbers of age-overlapped summaries maintained in parallel. Of course only 'top' one is rendered during scraping.

Due to the huge overhead maybe it's good idea to keep current simple implementation as an option?

References: 0. Effective Computation of Biased Quantiles over Data Streams

1. Ruby Quantiles implementation
2. Golang Quantiles implementation
3. Common Lisp implementation
4. Golang sliding window implementation

[hairyhum]

So I found this: https://github.com/odo/quantile_estimator It's rather old but seems to build, at least on OTP-21. I'll take a look if it's possible to integrate to the summary metric.