[data.search.aggs] Use fields instead of _source in top_hits agg

src/plugins/data/common/search/aggs/metrics/top_hit.test.ts

@@ -133,28 +133,28 @@ describe('Top hit metric', () => {
   });
 
   it('should request the _source field', () => {
-    init({ field: '_source' });
-    expect(aggDsl.top_hits._source).toBeTruthy();
-    expect(aggDsl.top_hits.docvalue_fields).toBeUndefined();
+    init({ fieldName: '_source' });
+    expect(aggDsl.top_hits._source).toBe(true);
+    expect(aggDsl.top_hits.fields).toBeUndefined();
   });
 
-  it('requests both source and docvalues_fields for non-text aggregatable fields', () => {
+  it('requests fields for non-text aggregatable fields', () => {
     init({ fieldName: 'bytes', readFromDocValues: true });
-    expect(aggDsl.top_hits._source).toBe('bytes');
-    expect(aggDsl.top_hits.docvalue_fields).toEqual([{ field: 'bytes' }]);
+    expect(aggDsl.top_hits._source).toBe(false);
+    expect(aggDsl.top_hits.fields).toEqual([{ field: 'bytes' }]);
   });
 
-  it('requests both source and docvalues_fields for date aggregatable fields', () => {
+  it('requests fields for date aggregatable fields', () => {
     init({ fieldName: '@timestamp', readFromDocValues: true, fieldType: KBN_FIELD_TYPES.DATE });
 
-    expect(aggDsl.top_hits._source).toBe('@timestamp');
-    expect(aggDsl.top_hits.docvalue_fields).toEqual([{ field: '@timestamp', format: 'date_time' }]);
+    expect(aggDsl.top_hits._source).toBe(false);
+    expect(aggDsl.top_hits.fields).toEqual([{ field: '@timestamp', format: 'date_time' }]);
   });
 
-  it('requests just source for aggregatable text fields', () => {
+  it('requests fields for aggregatable text fields', () => {
     init({ fieldName: 'machine.os' });
-    expect(aggDsl.top_hits._source).toBe('machine.os');
-    expect(aggDsl.top_hits.docvalue_fields).toBeUndefined();
+    expect(aggDsl.top_hits._source).toBe(false);
+    expect(aggDsl.top_hits.fields).toEqual([{ field: 'machine.os' }]);
   });
 
   describe('try to get the value from the top hit', () => {

src/plugins/data/common/search/aggs/metrics/top_hit.ts

@@ -78,8 +78,8 @@ export const getTopHitMetricAgg = () => {
               },
             };
           } else {
-            if (field.readFromDocValues) {
-              output.params.docvalue_fields = [
+            if (field.name !== '_source') {
+              output.params.fields = [
                 {
                   field: field.name,
                   // always format date fields as date_time to avoid
@@ -89,7 +89,7 @@ export const getTopHitMetricAgg = () => {
                 },
               ];
             }
-            output.params._source = field.name === '_source' ? true : field.name;
+            output.params._source = field.name === '_source';
           }
         },
       },