### 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/AsyncSearchResponse.java
/**
     * Returns the current {@link SearchResponse} or <code>null</code> if not available.
     *
     * See {@link #isPartial()} to determine whether the response contains partial or complete
     * results.
     */
    public SearchResponse getSearchResponse() {
        return searchResponse;
    }
    /**
     * Returns the failure reason or null if the query is running or has completed normally.
     */
    public Exception getFailure() {
        return error;
    }
    /**
     * Returns <code>true</code> if the {@link SearchResponse} contains partial
     * results computed from a subset of the total shards.
     */
    public boolean isPartial() {
        return isPartial;
    }
    /**
     * Whether the search is still running in the cluster.
     *
     * A value of <code>false</code> indicates that the response is final
     * even if {@link #isPartial()} returns <code>true</code>. In such case,
     * the partial response represents the status of the search before a
     * non-recoverable failure.
     */