code: private static double hotness(SSTableReader sstr)
      {
          // system tables don't have read meters, just use 0.0 for the hotness
          return sstr.getReadMeter() == null ? 0.0 : sstr.getReadMeter().twoHourRate() / sstr.estimatedKeys();
      }
docs: Returns the reads per second per key for this sstable, or 0.0 if the sstable has no read meter