### Description

Currently async_search get or status response include the following info about async_search:

```
  "is_partial" : true, 
  "is_running" : true, 
  "start_time_in_millis" : 1583945890986,
  "expiration_time_in_millis" : 1584377890986,
```

In Kibana we would also like to have something like `completion_time` that would indicate time when the search was completed. It doesn't have to be precise, having some margin of error would be fine. 

**Some context:**

Currently Kibana uses background tasks to monitor completion of searches and it saves the completion time and then later shows it to users when they restore the search. Now we are simplifying the implementation of searches restoration on Kibana side and removing search monitoring in background tasks [(RFC here)](https://docs.google.com/document/d/1xpvW0xHmdm6w6EyYzYd01JLhiPFWF1Ij-tcJxCRLO7U/edit#) so Kibana can't longer track on its side the completion time. 

x-pack/plugin/async-search/src/main/java/org/elasticsearch/xpack/search/MutableSearchResponse.java
);
            this.clusters = newClusters;
            logger.debug("Updating Clusters info to indicate that the local cluster search has completed: {}", newClusters);
        }
    }
    /**
     * Updates the response with the final {@link SearchResponse} once the
     * search is complete.
     */
    synchronized void updateFinalResponse(SearchResponse response, boolean ccsMinimizeRoundtrips) {
        failIfFrozen();
        assert shardsInResponseMatchExpected(response, ccsMinimizeRoundtrips)
            : getShardsInResponseMismatchInfo(response, ccsMinimizeRoundtrips);
        this.responseHeaders = threadContext.getResponseHeaders();
        this.finalResponse = response;
        this.isPartial = false;
        this.frozen = true;
    }
    /**
     * Updates the response with a fatal failure. This method preserves the partial response
     * received from previous updates
     */
    synchronized void updateWithFailure(ElasticsearchException exc) {
        failIfFrozen();
        
        this.responseHeaders = threadContext.getResponseHeaders();
        
        
        this.isPartial = true;
        this.failure = exc;