Add clarification for alerting on custom metrics

The custom metric should already be reported, because it's not possible to define an alert rule on a non-existent custom metric

Author: harelbr

articles/azure-monitor/platform/alerts-metric-create-templates.md

@@ -5,7 +5,7 @@ author: harelbr
 services: azure-monitor
 ms.service: azure-monitor
 ms.topic: conceptual
-ms.date: 2/17/2020
+ms.date: 2/24/2020
 ms.author: harelbr
 ms.subservice: alerts
 ---
@@ -1245,7 +1245,7 @@ You can use the following template to create a more advanced static threshold me
 
 To learn more about custom metrics in Azure Monitor, see [Custom metrics in Azure Monitor](https://docs.microsoft.com/azure/azure-monitor/platform/metrics-custom-overview).
 
-When creating an alert rule on a custom metric, you need to specify both the metric name and the metric namespace.
+When creating an alert rule on a custom metric, you need to specify both the metric name and the metric namespace. You should also make sure that the custom metric is already being reported, as you cannot create an alert rule on a custom metric that doesn't yet exist.
 
 Save the json below as customstaticmetricalert.json for the purpose of this walkthrough.