Number of non-nan entries in each rolling window.

def _counts(self):
        """ Number of non-nan entries in each rolling window. """

        rolling_dim = utils.get_temp_dimname(self.obj.dims, '_rolling_dim')
        # We use False as the fill_value instead of np.nan, since boolean
        # array is faster to be reduced than object array.
        # The use of skipna==False is also faster since it does not need to
        # copy the strided array.
        counts = (self.obj.notnull()
                  .rolling(center=self.center, **{self.dim: self.window})
                  .construct(rolling_dim, fill_value=False)
                  .sum(dim=rolling_dim, skipna=False))
        return counts