Document monitoring with Prometheus

JanusGraph has a [chapter about monitoring](https://docs.janusgraph.org/latest/monitoring.html) that describes the different monitoring solutions supported by JanusGraph out of the box.
While not directly supported, it is still possible without much effort to provide metrics from JanusGraph to Prometheus via JanusGraph's JMX Reporter and the [Prometheus JMX Exporter](https://github.com/prometheus/jmx_exporter).

Since [Prometheus seems to be the most prominent metrics solution nowadays](https://trends.google.com/trends/explore?cat=5&date=today%205-y&q=prometheus,ganglia,graphite), it would be good to have that covered in the docs so not every user has to figure out the best approach on his own.

As a follow-up to this issue, providing a dashboard for Grafana could be a good idea but that should probably get its own issue to limit the scope of this issue.

Start with the existing monitoring chapter at docs.janusgraph.org/latest/monitoring.html, then review JanusGraph's JMX Reporter and the Prometheus JMX Exporter documentation. Document the supported approach for exposing JanusGraph metrics to Prometheus, while keeping a Grafana dashboard out of scope.

Written by the indexing model from the issue text.