docs(monitors): heartbeat monitors and metric alerts guides; mask ping tokens in bundled nginx log

docs/vantage/notification-channels.md

@@ -30,6 +30,9 @@ Posts JSON to a URL you choose.
 }
 ```
 
+Alerts from a [metric alert](./metric-alerts.md) add `"server_name"` with the
+server that breached. Other monitor types leave it out.
+
 Any response of 300 or above counts as a delivery failure. The request times out
 after 10 seconds.