Added Kannel exporter (prometheus#944)

Author: apostvav

content/docs/instrumenting/exporters.md

@@ -136,6 +136,7 @@ wide variety of JVM-based applications, for example [Kafka](http://kafka.apache.
    * [Dovecot exporter](https://github.com/kumina/dovecot_exporter)
    * [Jenkins exporter](https://github.com/lovoo/jenkins_exporter)
    * [JIRA exporter](https://github.com/AndreyVMarkelov/jira-prometheus-exporter)
+   * [Kannel exporter](https://github.com/apostvav/kannel_exporter)
    * [Kemp LoadBalancer exporter](https://github.com/giantswarm/prometheus-kemp-exporter)
    * [Meteor JS web framework exporter](https://atmospherejs.com/sevki/prometheus-exporter)
    * [Minecraft exporter module](https://github.com/Baughn/PrometheusIntegration)