Hi Sylwester,

By default Python functions starting with _ are considered private methods. For more information have a look at https://stackoverflow.com/questions/1547145/defining-private-module-functions-in-python
In your case you can simply change the button name from _get_coords_from_address to get_coords_from_address (remove the _). If you do the same on the Python side it will work fine.
By removing the underscore Odoo will no longer see it as a private method, which caused your initial error.
On a sidenote, you're using the @api.one decorator which is pretty much deprecated. It is better to use @api.multi in combination with self.ensure_one. Your code shoud look like this in XML:

<button string="Map Coordinates" type="object" name="get_coords_from_address" class="oe_stat_button"/>

The Python:

@api.multi
def get_coords_from_address(self):
    # This will make sure that you have one record instead of multiple (singleton)
    self.ensure_one()
    if self.full_address != None:
        lat_long = geocoder.google(self.full_address)
        # self.lat_long = lat_long.latlng
    self.write({'lat_long': lat_long.latlng}) 

Regards,
Yenthe