Add mixin for Ruby Rack applications. (grafana#447)

This mixin includes a dashboard to monitor Ruby Rack applications using
the default middleware collector from Prometheus's client library.

The dashboard follows RED method and shows rate of http requests,
requests duration and rate of errors (5xx response codes).

ruby-mixin/README.md

@@ -0,0 +1,16 @@
+# Ruby Mixin
+
+_This mixin is a work in progress. We aim for it to become a good role model for dashboards eventually, but it's not there yet._
+
+Mixins are a collection of configurable, reusable Prometheus rules, alerts and/or Grafana dashboards for a particular system, usually created by experts in that system. By applying them to Prometheus and Grafana, you can quickly set up appropriate monitoring for your systems.
+
+This mixin is for Ruby applications, and contains a dashboard for visualizing the http metrics produced by the [Rack middleware collector](https://github.com/prometheus/client_ruby/blob/master/lib/prometheus/middleware/collector.rb) included with the Prometheus client library for Ruby.
+
+To use the mixin, you need to have `mixtool` and `jsonnetfmt` installed. If you have a working Go development environment, it's easiest to run the following:
+
+```bash
+$ go get github.com/monitoring-mixins/mixtool/cmd/mixtool
+$ go get github.com/google/go-jsonnet/cmd/jsonnetfmt
+```
+
+For more advanced uses of mixins, see https://github.com/monitoring-mixins/docs.