@Override public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks, long timeout,
      TimeUnit unit) throws InterruptedException {
    return delegate().invokeAll(wrap(tasks), timeout, unit);
  }
Used to implement a context propagating executor service which wraps tasks
zipkin-server/src/main/java/zipkin2/server/internal/WrappingExecutorService.java