add Kong into exporters list (#1342)

https://github.com/Kong/kong-plugin-prometheusSigned-off-by: Jens Bissinger <mail@jens-bissinger.de>

content/docs/instrumenting/exporters.md

@@ -216,6 +216,7 @@ separate exporters are needed:
    * [FreeBSD Kernel](https://www.freebsd.org/cgi/man.cgi?query=prometheus_sysctl_exporter&apropos=0&sektion=8&manpath=FreeBSD+12-current&arch=default&format=html)
    * [Grafana](http://docs.grafana.org/administration/metrics/)
    * [JavaMelody](https://github.com/javamelody/javamelody/wiki/UserGuideAdvanced#exposing-metrics-to-prometheus)
+   * [Kong](https://github.com/Kong/kong-plugin-prometheus)
    * [Kubernetes](https://github.com/kubernetes/kubernetes) (**direct**)
    * [Linkerd](https://github.com/BuoyantIO/linkerd)
    * [mgmt](https://github.com/purpleidea/mgmt/blob/master/docs/prometheus.md)