Add an event tracking callback to allow for inserting custom trackers

[kleinjoshuaa]

Sometimes we have other trackers that we like to 'piggy back' on to GA4 for tracking events.

It would be really cool to be able to do something like this:

tracker.addEventCallback((user_id, eventsParameters, persistentEventParameters, userProperties) => {
// add custom tracker code here
})

And then every time trackEvent is called this is called too