Update Get-AzMetricDefinition to include a note (link) to list of supported metrics

Author: awsles

src/Monitor/Monitor/help/Get-AzMetricDefinition.md

@@ -219,6 +219,8 @@ This cmdlet supports the common parameters: -Debug, -ErrorAction, -ErrorVariable
 ### Microsoft.Azure.Commands.Insights.OutputClasses.PSMetricDefinition
 
 ## NOTES
+More information about the supported metrics may be found at:
+https://docs.microsoft.com/en-us/azure/azure-monitor/platform/metrics-supported
 
 ## RELATED LINKS