Some sources will directly ground them, some will put like a 100 ohm resistor in between. Some sources suggest leaving it floating.
It is not as if it makes a functional difference. Not that it protects anything from ESD in any meaningful way.
On the Leonardo it look like it's ... hm. A "soft ground" through resistor.
On the Micro it is a hard ground -- they literally go directly to the same flood fill.
The Due also have two hard grounds.