Update README for Prometheus Client Output (#2452)

plugins/outputs/prometheus_client/README.md

 # Prometheus Client Service Output Plugin
 
-This plugin starts a Prometheus Client, listening on a port defined in the
-configuration file.
+This plugin starts a [Prometheus](https://prometheus.io/) Client, it exposes all metrics on `/metrics` to be polled by a Prometheus server.
 
-It exposes all metrics on `/metrics` to be polled by a Prometheus server.
+## Configuration
+
+```
+# Publish all metrics to /metrics for Prometheus to scrape
+[[outputs.prometheus_client]]
+  # Address to listen on
+  listen = ":9126"
+
+  # Expiration interval for each metric. 0 == no expiration
+  expiration_interval = "60s"
+```