Add support for summary metric type

[alSergey]

Hi, we are really missing support for the Summary metric type. I saw this issue, where it was mentioned that Histogram metrics are more useful than Summary and should be used instead. Unfortunately, our company has a metrics standard, and every service must write metrics according to this standard. We have some metrics that must specifically be of the Summary type. Writing Histogram metrics with the same name instead of Summary is not an option because our Grafana dashboards rely on the Summary metric labels.

[alSergey]

@knyar hi, is it possible to add the summary metric type?

[knyar]

It's not something I personally have a need for, but I will be happy to review a PR adding support for summary metrics.