Explore metrics: Fix warning message for limiting metrics (#93817)

don't show undefined error

public/app/features/trails/MetricSelect/MetricSelectScene.tsx

@@ -287,8 +287,7 @@ export class MetricSelectScene extends SceneObjectBase<MetricSelectSceneState> i
       }
 
       if (missingOtelTargets) {
-        metricNamesWarning +=
-          'The list of metrics is not complete. Select more OTel resource attributes to see a full list of metrics.';
+        metricNamesWarning = `${metricNamesWarning ?? ''} The list of metrics is not complete. Select more OTel resource attributes to see a full list of metrics.`;
       }
 
       let bodyLayout = this.state.body;