Add default value for mode for rate agg

public/app/plugins/datasource/elasticsearch/components/QueryEditor/MetricAggregationsEditor/SettingsEditor/index.tsx

@@ -141,7 +141,7 @@ export const SettingsEditor: FunctionComponent<Props> = ({ metric, previousMetri
               id={`ES-query-${query.refId}_metric-${metric.id}-mode`}
               onChange={(e) => dispatch(changeMetricSetting(metric, 'mode', e.value))}
               options={rateAggModeOptions}
-              value={metric.settings?.unit}
+              value={metric.settings?.unit || metricAggregationConfig['rate'].defaults.settings?.mode}
             />
           </InlineField>
         </>

public/app/plugins/datasource/elasticsearch/components/QueryEditor/MetricAggregationsEditor/utils.ts

@@ -245,7 +245,11 @@ export const metricAggregationConfig: MetricsConfiguration = {
     hasSettings: true,
     supportsInlineScript: false,
     hasMeta: false,
-    defaults: {},
+    defaults: {
+      settings: {
+        mode: 'sum',
+      },
+    },
   },
 };