ad_monitroing.js

var nr_monitor_ads = function(){
		var convertToString = ['advertiserId', 'campaignId', 'creativeId', 'lineItemId', 'labelIds', 'sourceAgnosticCreativeId', 'sourceAgnosticLineItemId'];
		var NR_SUBMIT_PAGE_ACTION = function(ev, obj) {
			if (typeof window.newrelic != 'undefined' && typeof window.newrelic.addPageAction == 'function') {
				for (i in obj) {
					if (convertToString.indexOf(i) > -1) {
						obj[i] = String(obj[i]);	
					}
				}
				window.newrelic.addPageAction(ev, obj)
			}
		};
		var NR_ONLOAD = Date.now();
		var NR_LOADED_TIMESTAMP = Date.now();
		var ready = false;
		var containers = {};

		var wrapFn = function(fn){
		  return function(){
		    try{
		      if (ready) {
		      	NR_LOADED_TIMESTAMP = Date.now();
		      }
		      if (arguments && arguments.length > 0) {
		      	var container = arguments[0];
		      	containers[container] = Date.now();
		      } 
		      return fn.apply(this, arguments);
		    }catch(ex){
		      ErrorHandler.Exception(ex);
		    }
		  };
		};


		function wrapRefresh() {
			googletag.display = wrapFn(googletag.display);
		};

		
		window.googletag = window.googletag || {};
		wrapRefresh();
		googletag.cmd = googletag.cmd || [];

		googletag.cmd.push(detectAdLoad);

		function detectAdLoad() {
		    if(window.googletag && googletag.pubadsReady) {
		        ready = true;
		        var NR_PUBADSREADY = Date.now();
		        
		        googletag.pubads().addEventListener('slotRenderEnded', function(event) {
		        	if (event && event.slot) {
		        		var container = event.slot.getSlotElementId();
			        	NR_SUBMIT_PAGE_ACTION('pubAdsRendered', {
			        		readyTime: Date.now(),
			        		advertiserId: event.advertiserId,
			        		campaignId: event.campaignId,
			        		creativeId: event.creativeId,
			        		isEmpty: event.isEmpty ? 'empty' : 'notEmpty',
			        		lineItemId: event.lineItemId,
			        		labelIds: event.labelIds,
			        		elementId: event.slot.getSlotElementId(),
			        		serviceName: event.serviceName,
			        		width: (event.size && event.size.length > 0) ? event.size[0] : null,
			        		height: (event.size && event.size.length > 1) ? event.size[1] : null,
			        		slotContentChanged: event.slotContentChanged,
			        		sourceAgnosticCreativeId: event.sourceAgnosticCreativeId,
			        		sourceAgnosticLineItemId: event.sourceAgnosticLineItemId,
			        		slotId: (event.slot && event.slot['$']) ? event.slot['$'] : null,
			        		onloadTime: NR_LOADED_TIMESTAMP,
			        		adsReady: NR_PUBADSREADY,
			        		NR_ONLOAD: NR_ONLOAD,
			        		displayTime: (containers[container]) ? containers[container] : NR_LOADED_TIMESTAMP
			        	});	
		        	}
		        	
		        });

		        googletag.pubads().addEventListener('impressionViewable', function(event) {
		        	if (event && event.slot) {
			        	var container = event.slot.getSlotElementId();
			        	NR_SUBMIT_PAGE_ACTION('pubAdsViewable', {
			        		readyTime: Date.now(),
			        		serviceName: event.serviceName,
			        		elementId: event.slot.getSlotElementId(),
			        		slotId: (event.slot && event.slot['$']) ? event.slot['$'] : '',
			        		advertiserId: (event.slot && event.slot['H'] && event.slot['H'].advertiserId) ? event.slot['H'].advertiserId : null,
			        		campaignId: (event.slot && event.slot['H'] && event.slot['H'].campaignId) ? event.slot['H'].campaignId : null,
			        		creativeId: (event.slot && event.slot['H'] && event.slot['H'].creativeId) ? event.slot['H'].creativeId : null,
			        		lineItemId: (event.slot && event.slot['H'] && event.slot['H'].lineItemId) ? event.slot['H'].lineItemId : null,
			        		labelIds: (event.slot && event.slot['H'] && event.slot['H'].labelIds) ? event.slot['H'].labelIds : null,
			        		onloadTime: NR_LOADED_TIMESTAMP,
			        		adsReady: NR_PUBADSREADY,
			        		NR_ONLOAD: NR_ONLOAD,
			        		displayTime: (containers[container]) ? containers[container] : NR_LOADED_TIMESTAMP
			        	});
					}
		        });
		    }
		}
};

if (window.addEventListener)  {
	window.addEventListener('load', nr_monitor_ads, false);
} else if (window.attachEvent) {
  window.attachEvent('onload', nr_monitor_ads);
}

I've found this documentation from google:
https://developers.google.com/doubleclick-gpt/reference#googletag.events.slotVisibilityChangedEvent

I think this is a really interesting event, as it exposes the % of adbanner shown.

Well look what we have here... ❤️