Add RabbitMQ to software that exposes Prometheus metrics (prometheus#1414)

As of v3.8.0, RabbitMQ exposes Prometheus metrics natively, no separate
exporter needed. To learn more, see the official Prometheus & Grafana
guide: https://rabbitmq.com/prometheus.html

We - @rabbitmq - maintain Grafana dashboards on grafana.com:
https://grafana.com/orgs/rabbitmq

Signed-off-by: Gerhard Lazu <gerhard@lazu.co.uk>

Author: gerhard

content/docs/instrumenting/exporters.md

@@ -239,6 +239,7 @@ separate exporters are needed:
    * [Netdata](https://github.com/firehol/netdata)
    * [Pretix](https://pretix.eu/)
    * [Quobyte](https://www.quobyte.com/) (**direct**)
+   * [RabbitMQ](https://rabbitmq.com/prometheus.html)
    * [RobustIRC](http://robustirc.net/)
    * [ScyllaDB](http://github.com/scylladb/scylla)
    * [Skipper](https://github.com/zalando/skipper)