Add MemoryUsagePercentage metric to feature availability table

Author: maud-lv

articles/managed-grafana/known-limitations.md

@@ -49,6 +49,7 @@ Some Azure Managed Grafana features aren't available in Azure Government and Mic
 | Team sync with Microsoft Entra ID |      Preview     |                     Preview                    |
 | Enterprise plugins                |   Not supported  |                  Not supported                 |
 | Essential plan                    |     Supported    |                  Not supported                 |
+| MemoryUsagePercentage metric      |   Not supported  |                  Not supported                 |
 
 ## Throttling limits and quotas