docs: Downsampler downsampler(final long start_time,
                            final long end_time,
                            final long interval_ms,
                            final Aggregator downsampler,
                            final FillPolicy fill_policy) {
      if (FillPolicy.NONE == fill_policy) {
        // The default downsampler simply skips missing intervals, causing the
        // span group to linearly interpolate.
        return new Downsampler(spanIterator(), interval_ms, downsampler);
      } else {
        // Otherwise, we need to instantiate a downsampler that can fill missing
        // intervals with special values.
        return new FillingDownsampler(spanIterator(), start_time, end_time,
          interval_ms, downsampler, fill_policy);
      }
    }