Document real world examples of using relabel_config

WRT http://prometheus.io/docs/operating/configuration/#%3Crelabel_config%3E

The documentation for relabel_config is very clinical and hard to understand. (At least it is accurate, as far as I can tell.)

In both relabel_configs and metric_relabel_configs some real-world examples would go a long way. (In fact, relabel_configs doesn't even have a paragraph. metric_relabel_configs says "One use for this is to blacklist time series that are too expensive to ingest" but gives no hint at how to accomplish that.)

It would be great if the examples gave some typical uses for relabeling, and included examples of relabeling a kubernetes_sd_config or similar.

I found the relabelling much easier to understand after I found this blog post which contained a few sparse examples: https://prometheus.io/blog/2015/06/01/advanced-service-discovery/ but, being a blog post instead of docs, it's already out of date.

Start with the relabel_config documentation at the linked configuration section, covering both relabel_configs and metric_relabel_configs. Review the linked advanced service discovery blog for candidate examples, then check them against current documentation for Kubernetes service discovery. Done means the documentation explains typical relabeling uses with accurate, current examples.