consume () {
    return this._result.pipe(
      mergeMap(
        result =>
          new Observable(summaryObserver =>
            this._startStreaming({ result, summaryObserver })
          )
      )
    )
  }
Returns an observable that exposes a single item of {@link ResultSummary} that is generated by
the server after the streaming of the executing query is completed.

*Subscribing to this stream before subscribing to records() stream causes the results to be discarded on the server.*

@public
@returns {Observable<ResultSummary>} - An observable stream (with exactly one element) of result summary.