The details say that the icon is:
https://developers.google.com/maps/documentation/javascript/marker-clustering
https://developers.google.com/maps/documentation/javascript/examples/markerclusterer/m
But now I'm extending the Google Maps forge component and I can't find this icon, but it appears in the app. Anyone did this before?
The icon is
https://developers.google.com/maps/documentation/javascript/examples/markerclusterer/m1.png
Google appends 1.png, 2.png, 3.png, 4.png, 5.png to the images, depending on the zoom level.
The image path must be Image_Name, and in your resources you must have Image_Name1.png,Image_Name2.png,Image_Name3.png,Image_Name4.png,Image_Name5.png