resolves elastic#58905

The current index threshold alert uses a `size` limit on term aggregation, when used, but does not sort the buckets, so it's just using descending count on the grouped buckets as the sort to determine what to return.

The watcher API for the index threshold notes this as "top N of", implying a sort.

This PR applies sorting when the using `groupBy: top`, and the `aggType != count`.  For count, ES is already sorting the way we want.

The sort is calculated as a separate agg beside the date_range aggregation, which is the same metrics agg specified in the query - `aggType(aggField)`.  This field is then referenced in a new `order` property in the terms agg, using 'asc' sorting for `min`, and `desc` sorting for `avg`, `max`, and `sum`.

This doesn't change the shape of the output at all, just changes which term buckets will be returned, if there are more term buckets than requested with the `termSize` parameter.

x-pack/plugins/alerting_builtins/server/alert_types/index_threshold/lib/time_series_query.ts

@@ -77,6 +77,15 @@ export async function timeSeriesQuery(
         },
       },
     };
+
+    // if not count add an order
+    if (!isCountAgg) {
+      const sortOrder = aggType === 'min' ? 'asc' : 'desc';
+      aggParent.aggs.groupAgg.terms.order = {
+        sortValueAgg: sortOrder,
+      };
+    }
+
     aggParent = aggParent.aggs.groupAgg;
   }
 
@@ -89,6 +98,16 @@ export async function timeSeriesQuery(
       },
     },
   };
+
+  // if not count, add a sorted value agg
+  if (!isCountAgg) {
+    aggParent.aggs.sortValueAgg = {
+      [aggType]: {
+        field: aggField,
+      },
+    };
+  }
+
   aggParent = aggParent.aggs.dateAgg;
 
   // finally, the metric aggregation, if requested
@@ -106,13 +125,20 @@ export async function timeSeriesQuery(
   const logPrefix = 'indexThreshold timeSeriesQuery: callCluster';
   logger.debug(`${logPrefix} call: ${JSON.stringify(esQuery)}`);
 
+  // note there are some commented out console.log()'s below, which are left
+  // in, as they are VERY useful when debugging these queries; debug logging
+  // isn't as nice since it's a single long JSON line.
+
+  // console.log('time_series_query.ts request\n', JSON.stringify(esQuery, null, 4));
   try {
     esResult = await callCluster('search', esQuery);
   } catch (err) {
+    // console.log('time_series_query.ts error\n', JSON.stringify(err, null, 4));
     logger.warn(`${logPrefix} error: ${JSON.stringify(err.message)}`);
     throw new Error('error running search');
   }
 
+  // console.log('time_series_query.ts response\n', JSON.stringify(esResult, null, 4));
   logger.debug(`${logPrefix} result: ${JSON.stringify(esResult)}`);
   return getResultFromEs(isCountAgg, isGroupAgg, esResult);
 }

x-pack/test/alerting_api_integration/spaces_only/tests/alerting/builtin_alert_types/index_threshold/time_series_query_endpoint.ts

@@ -196,14 +196,6 @@ export default function timeSeriesQueryEndpointTests({ getService }: FtrProvider
 
       const expected = {
         results: [
-          {
-            group: 'groupA',
-            metrics: [
-              [START_DATE_MINUS_2INTERVALS, 4 / 1],
-              [START_DATE_MINUS_1INTERVALS, (4 + 2) / 2],
-              [START_DATE_MINUS_0INTERVALS, (4 + 2 + 1) / 3],
-            ],
-          },
           {
             group: 'groupB',
             metrics: [
@@ -212,12 +204,50 @@ export default function timeSeriesQueryEndpointTests({ getService }: FtrProvider
               [START_DATE_MINUS_0INTERVALS, (5 + 3 + 2) / 3],
             ],
           },
+          {
+            group: 'groupA',
+            metrics: [
+              [START_DATE_MINUS_2INTERVALS, 4 / 1],
+              [START_DATE_MINUS_1INTERVALS, (4 + 2) / 2],
+              [START_DATE_MINUS_0INTERVALS, (4 + 2 + 1) / 3],
+            ],
+          },
         ],
       };
 
       expect(await runQueryExpect(query, 200)).eql(expected);
     });
 
+    it('should return correct sorted group for average', async () => {
+      const query = getQueryBody({
+        aggType: 'avg',
+        aggField: 'testedValue',
+        groupBy: 'top',
+        termField: 'group',
+        termSize: 1,
+        dateStart: START_DATE_MINUS_2INTERVALS,
+        dateEnd: START_DATE_MINUS_0INTERVALS,
+      });
+      const result = await runQueryExpect(query, 200);
+      expect(result.results.length).to.be(1);
+      expect(result.results[0].group).to.be('groupB');
+    });
+
+    it('should return correct sorted group for min', async () => {
+      const query = getQueryBody({
+        aggType: 'min',
+        aggField: 'testedValue',
+        groupBy: 'top',
+        termField: 'group',
+        termSize: 1,
+        dateStart: START_DATE_MINUS_2INTERVALS,
+        dateEnd: START_DATE_MINUS_0INTERVALS,
+      });
+      const result = await runQueryExpect(query, 200);
+      expect(result.results.length).to.be(1);
+      expect(result.results[0].group).to.be('groupA');
+    });
+
     it('should return an error when passed invalid input', async () => {
       const query = { ...getQueryBody(), aggType: 'invalid-agg-type' };
       const expected = {