code: static <C> Callable<C> withSpan(Span span, boolean endSpan, Callable<C> callable) {
      return new CallableInSpan<C>(span, callable, endSpan);
    }
docs: Wraps a {@link Callable} so that it executes with the {@code span} as the current {@code Span}.
  
  @param span the {@code Span} to be set as current.
  @param endSpan if {@code true} the returned {@code Runnable} will close the {@code Span}.
  @param callable the {@code Callable} to run in the {@code Span}.
  @return the wrapped {@code Callable}.