Add docs about Prom

book/src/SUMMARY.md

@@ -28,6 +28,7 @@
 	* [WebSocket](./websockets.md)
 * [Advanced Usage](./advanced.md)
     * [Database Configuration](./advanced_database.md)
+    * [Prometheus Metrics](./advanced_metrics.md)
 * [Contributing](./contributing.md)
 	* [Development Environment](./setup.md)
 * [FAQs](./faq.md)

book/src/advanced_metrics.md

@@ -0,0 +1,34 @@
+# Prometheus Metrics
+
+Lighthouse provides and extensive suite of metrics and monitoring in the
+[Prometheus](https://prometheus.io/docs/introduction/overview/) export format
+via a HTTP server built into Lighthouse.
+
+These metrics are generally consumed by a Prometheus server and displayed via a
+Grafana dashboard. These components are available in a docker-compose format at
+[sigp/lighthouse-metrics](https://github.com/sigp/lighthouse-metrics).
+
+## Beacon Node Metrics
+
+By default, these metrics are disabled but can be enabled with the `--metrics`
+flag. Use the `--metrics-address` and `--metrics-port` flags to customize the
+listening socket of the metrics server.
+
+### Example
+
+Start a beacon node with the metrics server enabled:
+
+```bash
+lighthouse bn --metrics
+```
+
+Check to ensure that the metrics are available on the default port:
+
+```bash
+curl localhost:5054/metrics
+```
+
+## Validator Client Metrics
+
+The validator client does not *yet* expose metrics, however this functionality
+is expected to be implemented in late-September 2020.