Add Flink to Software exposing Prometheus metrics (#1037)

Signed-off-by: Juan Paulo Gutierrez <juanpaulo.gutierrez@gmail.com>

content/docs/instrumenting/exporters.md

@@ -182,6 +182,7 @@ separate exporters are needed:
    * [Docker Daemon](https://docs.docker.com/engine/reference/commandline/dockerd/#daemon-metrics)
    * [Doorman](https://github.com/youtube/doorman) (**direct**)
    * [Etcd](https://github.com/coreos/etcd) (**direct**)
+   * [Flink](https://github.com/apache/flink)
    * [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/)
    * [Kubernetes](https://github.com/kubernetes/kubernetes) (**direct**)
@@ -214,4 +215,3 @@ practices.
    * Java/JVM: [Hystrix metrics publisher](https://github.com/ahus1/prometheus-hystrix)
    * Python-Django: [django-prometheus](https://github.com/korfuri/django-prometheus)
    * Node.js: [swagger-stats](https://github.com/slanatech/swagger-stats)
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