code: public IndexRequest expireAfter(final Long expireAfter, final TimeUnit timeUnit) {
          if (expireAfter == null) {
              this.expireAfterSeconds = null;
          } else {
              this.expireAfterSeconds = TimeUnit.SECONDS.convert(expireAfter, timeUnit);
          }
          return this;
      }
docs: Sets the time to live for documents in the collection
  
  @param expireAfter the time to live for documents in the collection
  @param timeUnit the time unit
  @return this
  @mongodb.driver.manual tutorial/expire-data TTL