Support ElasticSearch Metric endpoints

[mwringe]

ElasticSearch can expose metrics in a JSON based endpoint. It might be useful to gather metrics from this type of endpoint.

Not sure exactly if supporting this type of endpoint should be a high priority, or is even something we want do do, but this can be used as a reminder that we have this feature being requested.

The other option is for the logging guys to expose a prometheus endpoint instead, which HOSA can already directly read from.

[jmazzitelli]

FWIW: HOSA supports a generic "json" endpoint type (main use case was to support GoLang apps exposing metrics via expvar - you can see this in action via the go-expvar example) but if its a plain JSON document at the HTTP[s] endpoint, HOSA can read it and extract (as best it can) the metrics it finds.

Whether or not ElasticSearch's JSON is exposing metrics in a way HOSA can understand is another issue. See the docs for what HOSA supports when reading these generic JSON endpoints here: https://github.com/hawkular/hawkular-openshift-agent#generic-json-endpoints