Trying to install mediawiki and Cognate with php maintenance/install.php --with-extensions yields:
Creating tables for enabled extensions
[3014dd8e8d79d85637e602f9] [no req] BadMethodCallException from line 83 of includes/libs/rdbms/lbfactory/LBFactorySingle.php: Method is not supported.
Backtrace:
#0 extensions/Cognate/src/CognateHooks.php(118): Wikimedia\Rdbms\LBFactorySingle->getExternalLB(NULL)
#1 includes/Hooks.php(174): Cognate\CognateHooks::onLoadExtensionSchemaUpdates(MysqlUpdater)
#2 includes/Hooks.php(202): Hooks::callHook(string, array, array, NULL)
#3 includes/installer/DatabaseUpdater.php(127): Hooks::run(string, array)
#4 includes/installer/DatabaseUpdater.php(192): DatabaseUpdater->__construct(Wikimedia\Rdbms\DatabaseMysqli, boolean, NULL)
#5 includes/installer/DatabaseInstaller.php(312): DatabaseUpdater::newForDB(Wikimedia\Rdbms\DatabaseMysqli)
#6 includes/installer/Installer.php(1552): DatabaseInstaller->createExtensionTables(MysqlInstaller)
#7 includes/installer/CliInstaller.php(137): Installer->performInstallation(array, array)
#8 maintenance/install.php(125): CliInstaller->execute()
#9 maintenance/doMaintenance.php(94): CommandLineInstaller->execute()
#10 maintenance/install.php(175): require_once(string)
#11 {main}Repro: comment check experimental on the dummy change https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/Cognate/+/442292/