Add doc about third-party exporters.

content/docs/instrumenting/exporters.md

+---
+title: Exporters for third-party systems
+sort_rank: 4
+---
+
+# Exporters for third-party systems
+
+There are a number of libraries and servers which help in exporting existing
+metrics from third-party systems as Prometheus metrics. This is useful for
+cases where it is not feasible to instrument a given system with Prometheus
+metrics directly (for example, HAProxy or Linux system stats). The
+following is a list of existing third-party exporters:
+
+   * [node/system metrics exporter](https://github.com/prometheus/node_exporter)
+   * [HAProxy exporter](https://github.com/prometheus/haproxy_exporter)
+   * [AWS CloudWatch exporter](https://github.com/prometheus/cloudwatch_exporter)
+   * [StatsD bridge](https://github.com/prometheus/statsd_bridge)
+   * [JMX exporter](https://github.com/prometheus/jmx_exporter)
+   * [Hystrix metrics publisher](https://github.com/prometheus/hystrix)