EQL: correct time accounting for an intermediary response (#75804) (#76000)

This corrects the accounting of answer time reported in "took" attribute
of an intermediary EQL async response. This has been reported as a
negative value, since the delta mixes wall-clock time with VM time.

(The confusion is favoured by Task#getStartTime() returning a wall-clock
time, while Task#getStartTimeNanos() VM's time.)

Co-authored-by: Bogdan Pintea <[email protected]>

Author: elasticsearchmachine

x-pack/plugin/eql/qa/rest/src/yamlRestTest/resources/rest-api-spec/test/eql/10_basic.yml

@@ -309,6 +309,7 @@ setup:
 
   - is_true: id
   - set: {id: id}
+  - gte: {took: 0}
 
   - do:
       eql.get:

x-pack/plugin/eql/src/main/java/org/elasticsearch/xpack/eql/plugin/TransportEqlSearchAction.java

@@ -93,7 +93,7 @@ public void execute(EqlSearchRequest request, EqlSearchTask task, ActionListener
     @Override
     public EqlSearchResponse initialResponse(EqlSearchTask task) {
         return new EqlSearchResponse(EqlSearchResponse.Hits.EMPTY,
-            threadPool.relativeTimeInMillis() - task.getStartTime(), false, task.getExecutionId().getEncoded(), true, true);
+            TimeValue.nsecToMSec(System.nanoTime() - task.getStartTimeNanos()), false, task.getExecutionId().getEncoded(), true, true);
     }
 
     @Override