That's the etching that's been referred to. It's where ash melted into the ceramic glass. It's common to see it front and center near where the primary air enters the stove. That slight white haze needs to be cleaned off of the glass just as much if not more than other build-up.I've never had any dark stains on my glass, just that slight fly-ash kind of haze.
...One year in with my stove and I have an area of very, very slight opaqueness on the very centre area of the stove window. It's only visible when I am right there, close by, looking at it. It's ringed with an area that is slightly coloured, in the same way as oil creates colour in water.. It's not a mark that can be removed in any way. It's now part of the glass. Any idea what that might be?