code: public static ReadPreference primaryPreferred(final long maxStaleness, final TimeUnit timeUnit) {
          return new PrimaryPreferredReadPreference(Collections.<TagSet>emptyList(), maxStaleness, timeUnit);
      }
docs: Gets a read preference that forces reads to the primary if available, otherwise to a secondary.
  
  @param maxStaleness the max allowable staleness of secondaries. The minimum value is either 90 seconds, or the heartbeat frequency
  plus 10 seconds, whichever is greatest.
  @param timeUnit the time unit of maxStaleness
  @return ReadPreference which reads primary if available.
  @since 3.4
  @see TaggableReadPreference#getMaxStaleness(TimeUnit)