### 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/core/src/main/java/org/elasticsearch/xpack/core/search/action/AsyncStatusResponse.java
* Returns {@code true} if the search results are partial.
     * This could be either because async search hasn't finished yet,
     * or if it finished and some shards have failed.
     */
    public boolean isPartial() {
        return isPartial;
    }
    /**
     * Returns a timestamp when the search tasks started, in milliseconds since epoch.
     */
    public long getStartTime() {
        return startTimeMillis;
    }
    /**
     * Returns a timestamp when the search will be expired, in milliseconds since epoch.
     */
    @Override
    public long getExpirationTime() {
        return expirationTimeMillis;
    }
    /**
     * Returns the total number of shards the search is executed on.
     */
    public int getTotalShards() {
        return totalShards;
    }
    /**
     * Returns the number of successful shards the search was executed on.
     */
    public int getSuccessfulShards() {