bugfix for get-azmetric (#15367)

Author: VeryEarly

src/Monitor/Monitor/ChangeLog.md

@@ -19,6 +19,7 @@
 -->
 
 ## Upcoming Release
+* Fixed null reference bug for `Get-AzMetric` when `ResultType` set to "Metadata"
 * Fixed bug for `Add-AzAutoscaleSetting` not able to pipe result from `Get-AzAutoscaleSetting` [#13861]
 
 ## Version 2.5.0

src/Monitor/Monitor/OutputClasses/PSMetric.cs

@@ -62,7 +62,7 @@ public PSMetric(Metric metric)
             this.Unit = metric.Unit;
             this.Id = metric.Id;
             this.Type = metric.Type;
-            this.Data = ((metric.Timeseries != null && metric.Timeseries.Count > 0)? new PSMetricValuesCollection(metric.Timeseries[0].Data) : null);
+            this.Data = ((metric.Timeseries != null && metric.Timeseries.Count > 0 && metric.Timeseries[0].Data != null)? new PSMetricValuesCollection(metric.Timeseries[0].Data) : null);
             this.Timeseries = metric.Timeseries;
         }
     }