Including info about the new File System Usage metric

Author: btardif

articles/app-service/web-sites-monitor.md

@@ -63,6 +63,10 @@ You can increase or remove quotas from your app by upgrading your App Service pl
 
 ## Understand metrics
 
+> [!NOTE]
+> **File System Usage** is a new metric being rolled out globally, no data is expected unless you have been whitelisted for private preview.
+> 
+
 Metrics provide information about the app or the App Service plan's behavior.
 
 For an app, the available metrics are:
@@ -76,6 +80,7 @@ For an app, the available metrics are:
 | **Current Assemblies** | The current number of Assemblies loaded across all AppDomains in this application. |
 | **Data In** | The amount of incoming bandwidth consumed by the app, in MiB. |
 | **Data Out** | The amount of outgoing bandwidth consumed by the app, in MiB. |
+| **File System Usage** | Percentage of filesystem quota consumed by the app . |
 | **Gen 0 Garbage Collections** | The number of times the generation 0 objects are garbage collected since the start of the app process. Higher generation GCs include all lower generation GCs.|
 | **Gen 1 Garbage Collections** | The number of times the generation 1 objects are garbage collected since the start of the app process. Higher generation GCs include all lower generation GCs.|
 | **Gen 2 Garbage Collections** | The number of times the generation 2 objects are garbage collected since the start of the app process.|