Search Applications: use msearch to get indices counts (#163699)

We use the index stats API to get the index counts for each index that
belongs to a Search Application.
The index stats API is internal, we should be using the counts API.
`GET <index>/_count` is equivalent to `GET
<index>/_search?size=0&track_total_hits=true`.
In order to avoid doing a request for each index, we do a single msearch
request to get all index counts.

tested locally, the counts are retrieved correctly:

<img width="1507" alt="Screenshot 2023-08-11 at 13 50 08"
src="3a9d2628-5b85-45d4-91be-561b5a326495">

x-pack/plugins/enterprise_search/server/lib/search_applications/fetch_indices_stats.test.ts

@@ -15,6 +15,7 @@ describe('fetchIndicesStats lib function', () => {
         get: jest.fn(),
         stats: jest.fn(),
       },
+      msearch: jest.fn(),
     },
     asInternalUser: {},
   };
@@ -53,6 +54,18 @@ describe('fetchIndicesStats lib function', () => {
     },
   ];
 
+  const msearchResponse = {
+    responses: [
+      {
+        hits: {
+          total: {
+            value: 200,
+          },
+        },
+      },
+    ],
+  };
+
   beforeEach(() => {
     jest.clearAllMocks();
   });
@@ -60,6 +73,7 @@ describe('fetchIndicesStats lib function', () => {
   it('should return hydrated indices for all available and open indices', async () => {
     mockClient.asCurrentUser.indices.get.mockResolvedValueOnce(getAllAvailableIndexResponse);
     mockClient.asCurrentUser.indices.stats.mockResolvedValueOnce(indexStats);
+    mockClient.asCurrentUser.msearch.mockImplementationOnce(() => msearchResponse);
     await expect(
       fetchIndicesStats(mockClient as unknown as IScopedClusterClient, indices)
     ).resolves.toEqual(fetchIndicesStatsResponse);
@@ -77,6 +91,7 @@ describe('fetchIndicesStats lib function', () => {
     );
 
     mockClient.asCurrentUser.indices.stats.mockImplementationOnce(() => indexStats);
+    mockClient.asCurrentUser.msearch.mockImplementationOnce(() => msearchResponse);
 
     await expect(
       fetchIndicesStats(mockClient as unknown as IScopedClusterClient, [
@@ -95,6 +110,7 @@ describe('fetchIndicesStats lib function', () => {
   it('should return count : null, health: unknown for deleted index ', async () => {
     mockClient.asCurrentUser.indices.get.mockImplementationOnce(() => getAllAvailableIndexResponse);
     mockClient.asCurrentUser.indices.stats.mockImplementationOnce(() => indexStats);
+    mockClient.asCurrentUser.msearch.mockImplementationOnce(() => msearchResponse);
 
     await expect(
       fetchIndicesStats(mockClient as unknown as IScopedClusterClient, [

x-pack/plugins/enterprise_search/server/lib/search_applications/fetch_indices_stats.ts

@@ -5,6 +5,7 @@
  * 2.0.
  */
 
+import { MsearchRequestItem, SearchTotalHits } from '@elastic/elasticsearch/lib/api/types';
 import { IScopedClusterClient } from '@kbn/core-elasticsearch-server/src/client/scoped_cluster_client';
 
 import { EnterpriseSearchApplicationIndex } from '../../../common/types/search_applications';
@@ -17,15 +18,29 @@ export const fetchIndicesStats = async (
 ): Promise<EnterpriseSearchApplicationIndex[]> => {
   const indicesStats = await client.asCurrentUser.indices.stats({
     index: await availableIndices(client, indices),
-    metric: ['docs'],
   });
 
-  return indices.map((index) => {
-    const indexStats = indicesStats.indices?.[index];
+  const searches: MsearchRequestItem[] = [];
+  indices.forEach((index) => {
+    searches.push({ index });
+    searches.push({ size: 0, track_total_hits: true });
+  });
+  const msearchResponse = await client.asCurrentUser.msearch({ searches });
+  const docCounts = msearchResponse.responses.map((response) => {
+    if ('error' in response) {
+      return null;
+    }
+
+    const totalHits = response.hits.total as SearchTotalHits;
+    return totalHits.value;
+  });
+
+  return indices.map((indexName, number) => {
+    const indexStats = indicesStats.indices?.[indexName];
     return {
-      count: indexStats?.primaries?.docs?.count ?? null,
+      count: docCounts[number] ?? null,
       health: indexStats?.health ?? 'unknown',
-      name: index,
+      name: indexName,
     };
   });
 };