Configurable monitoring endpoint frequency (#3530)

## Issue Addressed

Closes #3514

## Proposed Changes

- Change default monitoring endpoint frequency to 120 seconds to fit with 30k requests/month limit.
- Allow configuration of the monitoring endpoint frequency using `--monitoring-endpoint-frequency N` where `N` is a value in seconds.

book/src/advanced_metrics.md

@@ -48,3 +48,39 @@ Check to ensure that the metrics are available on the default port:
 ```bash
 curl localhost:5064/metrics
 ```
+
+## Remote Monitoring
+
+Lighthouse has the ability to send a subset of metrics to a remote server for collection. Presently
+the main server offering remote monitoring is beaconcha.in. Instructions for setting this up
+can be found in beaconcha.in's docs:
+
+- <https://kb.beaconcha.in/beaconcha.in-explorer/mobile-app-less-than-greater-than-beacon-node>
+
+The Lighthouse flag for setting the monitoring URL is `--monitoring-endpoint`.
+
+When sending metrics to a remote server you should be conscious of security:
+
+- Only use a monitoring service that you trust: you are sending detailed information about
+  your validators and beacon node to this service which could be used to track you.
+- Always use an HTTPS URL to prevent the traffic being intercepted in transit.
+
+The specification for the monitoring endpoint can be found here:
+
+- <https://github.com/gobitfly/eth2-client-metrics>
+
+_Note: the similarly named [Validator Monitor](./validator-monitoring.md) feature is entirely
+independent of remote metric monitoring_.
+
+### Update Period
+
+You can adjust the frequency at which Lighthouse sends metrics to the remote server using the
+`--monitoring-endpoint-period` flag. It takes an integer value in seconds, defaulting to 60
+seconds.
+
+```
+lighthouse bn --monitoring-endpoint-period 60 --monitoring-endpoint "https://url"
+```
+
+Increasing the monitoring period between can be useful if you are running into rate limits when
+posting large amounts of data for multiple nodes.