Just log.debug() when a layer doesn't support len(collection).  A full warning isn't needed.
Just log.debug() when a layer doesn't support len(collection).  A full warning isn't needed.

perrygeo/Fiona,perrygeo/Fiona,Toblerity/Fiona,rbuffat/Fiona,Toblerity/Fiona,rbuffat/Fiona