Add and configure a Prometheus agent with metrics coming from http://druid.io/docs/0.9.2/operations/metrics.html. This requires two important actions:
- Decide with the team what metrics we want to expose.
- Create a custom ad-hoc Prometheus exporter. Druid offerts only what it is called an "Emitter", namely a way to push metrics to some target like an HTTP endpoint (via post), a file or Graphite. Prometheus requires the opposite, namely to poll a http interface that returns metrics formatted in a predefined way.
Druid does not currently use jmxtrans yet, so we'll skip the step entirely and go directly to Prometheus.