Rename heka-exporter and add new heka-exporter

We discuss renaming the old heka-exporter repo to heka-dashboard-exporter to avoid confusion here: https://github.com/docker-infra/heka_exporter/issues/2

content/docs/instrumenting/exporters.md

@@ -46,7 +46,8 @@ hosted outside of the Prometheus GitHub organization.
    * [CouchDB exporter](https://github.com/gesellix/couchdb-exporter)
    * [Django exporter](https://github.com/korfuri/django-prometheus)
    * [Google's mtail log data extractor](https://github.com/google/mtail)
-   * [Heka exporter](https://github.com/docker-infra/heka_exporter)
+   * [Heka dashboard exporter](https://github.com/docker-infra/heka_exporter)
+   * [Heka exporter](https://github.com/imgix/heka_exporter)
    * [HTTP(s)/TCP/ICMP blackbox prober](https://github.com/discordianfish/blackbox_prober)
    * [IoT Edison exporter](https://github.com/roman-vynar/edison_exporter)
    * [Jenkins exporter](https://github.com/RobustPerception/python_examples/tree/master/jenkins_exporter)