code: public long count(@Nullable final DBObject query, final ReadPreference readPreference) {
          return getCount(query, null, readPreference);
      }
docs: Get the count of documents in collection that would match a criteria.
  
  @param query          specifies the selection criteria
  @param readPreference {@link ReadPreference} to be used for this operation
  @return the number of documents that matches selection criteria
  @throws MongoException if the operation failed
  @mongodb.driver.manual reference/command/count/ Count