Update metrics-supported.md

Adding explicit date because build system is now putting in the date of the last PR update, which is incorrect for this article in most cases. 
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cc: @rboucher

Author: rboucher

articles/azure-monitor/essentials/metrics-supported.md

@@ -13,6 +13,8 @@ ms.author: robb
 > [!NOTE]
 > This list is largely auto-generated. Any modification made to this list via GitHub might be written over without warning. Contact the author of this article for details on how to make permanent updates.
 
+Date list was last updated: 2021-10-05.
+
 Azure Monitor provides several ways to interact with metrics, including charting them in the Azure portal, accessing them through the REST API, or querying them by using PowerShell or the Azure CLI. 
 
 This article is a complete list of all platform (that is, automatically collected) metrics currently available with the consolidated metric pipeline in Azure Monitor. Metrics changed or added after the date at the top of this article might not yet appear in the list. To query for and access the list of metrics programmatically, use the [2018-01-01 api-version](/rest/api/monitor/metricdefinitions). Other metrics not in this list might be available in the portal or through legacy APIs.