Add link to Gmail exporter (prometheus#1593)

* Add link to Gmail exporter

Signed-off-by: jamesread <contact@jwread.com>
Co-authored-by: jamesread <contact@jwread.com>

Author: jamesread

content/docs/instrumenting/exporters.md

@@ -125,6 +125,7 @@ wide variety of JVM-based applications, for example [Kafka](http://kafka.apache.
    * [Docker Cloud exporter](https://github.com/infinityworksltd/docker-cloud-exporter)
    * [Docker Hub exporter](https://github.com/infinityworksltd/docker-hub-exporter)
    * [GitHub exporter](https://github.com/infinityworksltd/github-exporter)
+   * [Gmail exporter](https://github.com/jamesread/prometheus-gmail-exporter/)
    * [InstaClustr exporter](https://github.com/fcgravalos/instaclustr_exporter)
    * [Mozilla Observatory exporter](https://github.com/Jimdo/observatory-exporter)
    * [OpenWeatherMap exporter](https://github.com/RichiH/openweathermap_exporter)