[Synthetics] Add msearch helper to Synthetics ES client (#191747)

## Summary

This is a nice-to-have as it simplifies future usage of the
Elasticsearch client's `msearch` API for handing multiple queries to ES
at the same time.

We could also refactor this to consolidate similar helpers in other
places, for instance APM has a very similar

Co-authored-by: Shahzad <shahzad31comp@gmail.com>

x-pack/plugins/observability_solution/synthetics/server/lib.test.ts

@@ -5,6 +5,7 @@
  * 2.0.
  */
 
+import { MsearchResponse } from '@elastic/elasticsearch/lib/api/types';
 import { SyntheticsEsClient } from './lib';
 import { savedObjectsClientMock, uiSettingsServiceMock } from '@kbn/core/server/mocks';
 import { elasticsearchClientMock } from '@kbn/core-elasticsearch-client-server-mocks';
@@ -22,6 +23,73 @@ describe('SyntheticsEsClient', () => {
     jest.clearAllMocks();
   });
 
+  describe('msearch', () => {
+    it('should call baseESClient.msearch with correct parameters', async () => {
+      esClient.msearch.mockResolvedValueOnce({
+        body: {
+          responses: [
+            { aggregations: { aggName: { value: 'str' } } },
+            { aggregations: { aggName: { value: 'str' } } },
+          ],
+        },
+      } as unknown as MsearchResponse);
+
+      const mockSearchParams = [
+        {
+          query: {
+            match_all: {},
+          },
+        },
+        {
+          query: {
+            match_all: {},
+          },
+        },
+      ];
+
+      const result = await syntheticsEsClient.msearch(mockSearchParams);
+
+      expect(esClient.msearch).toHaveBeenCalledWith(
+        {
+          searches: [
+            {
+              index: 'synthetics-*',
+              ignore_unavailable: true,
+            },
+            mockSearchParams[0],
+            {
+              index: 'synthetics-*',
+              ignore_unavailable: true,
+            },
+            mockSearchParams[1],
+          ],
+        },
+        { meta: true }
+      );
+
+      expect(result).toMatchInlineSnapshot(`
+        Object {
+          "responses": Array [
+            Object {
+              "aggregations": Object {
+                "aggName": Object {
+                  "value": "str",
+                },
+              },
+            },
+            Object {
+              "aggregations": Object {
+                "aggName": Object {
+                  "value": "str",
+                },
+              },
+            },
+          ],
+        }
+      `);
+    });
+  });
+
   describe('search', () => {
     it('should call baseESClient.search with correct parameters', async () => {
       const mockSearchParams = {

x-pack/plugins/observability_solution/synthetics/server/lib.ts

@@ -5,6 +5,10 @@
  * 2.0.
  */
 
+import {
+  MsearchMultisearchBody,
+  MsearchMultisearchHeader,
+} from '@elastic/elasticsearch/lib/api/types';
 import {
   ElasticsearchClient,
   SavedObjectsClientContract,
@@ -13,7 +17,7 @@ import {
 } from '@kbn/core/server';
 import chalk from 'chalk';
 import type * as estypes from '@elastic/elasticsearch/lib/api/typesWithBodyKey';
-import type { ESSearchResponse } from '@kbn/es-types';
+import type { ESSearchResponse, InferSearchResponseOf } from '@kbn/es-types';
 import { RequestStatus } from '@kbn/inspector-plugin/common';
 import { InspectResponse } from '@kbn/observability-plugin/typings/common';
 import { enableInspectEsQueries } from '@kbn/observability-plugin/common';
@@ -116,6 +120,33 @@ export class SyntheticsEsClient {
 
     return res;
   }
+
+  async msearch<
+    TDocument = unknown,
+    TSearchRequest extends estypes.SearchRequest = estypes.SearchRequest
+  >(
+    requests: MsearchMultisearchBody[]
+  ): Promise<{ responses: Array<InferSearchResponseOf<TDocument, TSearchRequest>> }> {
+    const searches: Array<MsearchMultisearchHeader | MsearchMultisearchBody> = [];
+    for (const request of requests) {
+      searches.push({ index: SYNTHETICS_INDEX_PATTERN, ignore_unavailable: true });
+      searches.push(request);
+    }
+
+    const results = await this.baseESClient.msearch(
+      {
+        searches,
+      },
+      { meta: true }
+    );
+
+    return {
+      responses: results.body.responses as unknown as Array<
+        InferSearchResponseOf<TDocument, TSearchRequest>
+      >,
+    };
+  }
+
   async count<TParams>(params: TParams): Promise<CountResponse> {
     let res: any;
     let esError: any;