code: public static MongoCredential createPlainCredential(final String userName, final String source, final char[] password) {
          return new MongoCredential(PLAIN, userName, source, password);
      }
docs: Creates a MongoCredential instance for the PLAIN SASL mechanism.
  
  @param userName the non-null user name
  @param source   the source where the user is defined.  This can be either {@code "$external"} or the name of a database.
  @param password the non-null user password
  @return the credential
  
  @since 2.12
  @mongodb.server.release 2.6
  @mongodb.driver.manual core/authentication/#ldap-proxy-authority-authentication PLAIN