In T253121: MEP Client MediaWiki PHP, we implemented a simple PHP client that can produce to EventGate as EventServiceClient. However, PHP is weird, and some things that have already been solved in EventBus->send() (in the EventBus extension) are biting us here. Specifically, PHP is auto-serializing boolean values as integers. There may be other problems too.
Instead of fix them one by one, we should add a dependency on EventBus in EventLogging, use an EventBus.EventBusFactory to get an EventBus instance that will produce to eventgate-analytics-external, and just call $eventBus->send() from EventLogging::submit().
This will require that we declare eventgate-analytics-external in (e.g.) ProductionServices.php, as well as set up a local envoy proxy configs to forward from a local http port to eventgate-analytics-external, as we do for the EventGate instances that EventBus currently uses.