Extend Prombench for scalability tests

[sipian]

At promcon, people were interested in knowing on how to use prombench for scalability tests for their Prometheus infrastructures.

[krasi-georgiev]

Frederic’s idea for the scalability test (probably doesn't fit with our design) https://github.com/brancz/prometheus-scalability

a recent blog post on the topic https://www.percona.com/blog/2018/09/20/prometheus-2-times-series-storage-performance-analyses/