Merge pull request #1308 from slovdahl/add-envoy

Add Envoy to list of software exposing metrics

content/docs/instrumenting/exporters.md

@@ -203,6 +203,7 @@ separate exporters are needed:
    * [CRG Roller Derby Scoreboard](https://github.com/rollerderby/scoreboard) (**direct**)
    * [Docker Daemon](https://docs.docker.com/engine/reference/commandline/dockerd/#daemon-metrics)
    * [Doorman](https://github.com/youtube/doorman) (**direct**)
+   * [Envoy](https://www.envoyproxy.io/docs/envoy/latest/operations/admin.html#get--stats?format=prometheus)
    * [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)