Elasticsearch: Fix using multiple indexes with comma separated string (#71284)

* Revert "Elasticsearch: Use array of strings as index in backend queries (#67276)"

This reverts commit d0ced39847.

* updated tests

pkg/tsdb/elasticsearch/client/client.go

@@ -207,7 +207,7 @@ func (c *baseClientImpl) createMultiSearchRequests(searchRequests []*SearchReque
 			header: map[string]interface{}{
 				"search_type":        "query_then_fetch",
 				"ignore_unavailable": true,
-				"index":              c.indices,
+				"index":              strings.Join(c.indices, ","),
 			},
 			body:     searchReq,
 			interval: searchReq.Interval,