Add new search load metrics to SearchStats (#4731) (#4740)

* update specification for recent_search_load

* fix typo

(cherry picked from commit 9fc8173)

Co-authored-by: Dimitris Rempapis <[email protected]>

Author: github-actions[bot]

output/openapi/elasticsearch-openapi.json

@@ -272,6 +272,7 @@ export class SearchStats {
   suggest_time?: Duration
   suggest_time_in_millis: DurationValue<UnitMillis>
   suggest_total: long
+  recent_search_load?: double
   groups?: Dictionary<string, SearchStats>
 }