[ML] @kbn/ml-response-stream: Fix race condition related to throttling. (#162803)

Fixes a race condition in the case where a response stream finishes and
sets `isRunning` to `false`, but `useThrottle` didn't trigger it's last
update yet within the refresh rate. In the case of log rate analysis,
`isRunning` could be set to `false` too early and the UI wouldn't
consider later throttled updates (for example, setting `loaded=1` which
would result in inconsistent UI state).

The fix in this case is to return the unthrottled raw data instead of
the throttled one as soon as the stream finished.

x-pack/packages/ml/response_stream/client/use_fetch_stream.ts

@@ -129,7 +129,10 @@ export function useFetchStream<B extends object, R extends Reducer<any, any>>(
 
   return {
     cancel,
-    data: dataThrottled,
+    // To avoid a race condition where the stream already ended but `useThrottle` would
+    // yet have to trigger another update within the throttling interval, we'll return
+    // the unthrottled data once the stream is complete.
+    data: isRunning ? dataThrottled : data,
     dispatch,
     errors,
     isCancelled,