Elasticsearch: Allow omitting field when metric supports inline script (#32839)

* Elasticsearch: Allow omitting field when metric supports inline script

* Add tests for MetricEditor to show a None option

* Add tests for useFields hook

* Alerting: allow elasticsearch metrics without field

public/app/plugins/datasource/elasticsearch/hooks/useFields.ts

@@ -0,0 +1,53 @@
+import { MetricFindValue, SelectableValue } from '@grafana/data';
+import { BucketAggregationType } from '../components/QueryEditor/BucketAggregationsEditor/aggregations';
+import { useDatasource, useRange } from '../components/QueryEditor/ElasticsearchQueryContext';
+import {
+  isMetricAggregationType,
+  MetricAggregationType,
+} from '../components/QueryEditor/MetricAggregationsEditor/aggregations';
+
+type AggregationType = BucketAggregationType | MetricAggregationType;
+
+const getFilter = (aggregationType: AggregationType) => {
+  // For all metric types we want only numbers, except for cardinality
+  // TODO: To have a more configuration-driven editor, it would be nice to move this logic in
+  // metricAggregationConfig and bucketAggregationConfig so that each aggregation type can specify on
+  // which kind of data it operates.
+  if (isMetricAggregationType(aggregationType)) {
+    if (aggregationType !== 'cardinality') {
+      return 'number';
+    }
+
+    return void 0;
+  }
+
+  switch (aggregationType) {
+    case 'date_histogram':
+      return 'date';
+    case 'geohash_grid':
+      return 'geo_point';
+    default:
+      return void 0;
+  }
+};
+
+const toSelectableValue = ({ text }: MetricFindValue): SelectableValue<string> => ({
+  label: text,
+  value: text,
+});
+
+/**
+ * Returns a function to query the configured datasource for autocomplete values for the specified aggregation type.
+ * Each aggregation can be run on different types, for example avg only operates on numeric fields, geohash_grid only on geo_point fields.
+ * @param aggregationType the type of aggregation to get fields for
+ */
+export const useFields = (aggregationType: AggregationType) => {
+  const datasource = useDatasource();
+  const range = useRange();
+  const filter = getFilter(aggregationType);
+
+  return async () => {
+    const rawFields = await datasource.getFields(filter, range).toPromise();
+    return rawFields.map(toSelectableValue);
+  };
+};