In the line element
$ds=\frac{\partial s}{\partial x^1} dx^1+\frac{\partial s}{\partial x^2} dx^2$
(superscripts are indices, not powers)
the basis vectors are defined as $e_1=\frac{\partial s}{\partial x^1}$ and $e_2=\frac{\partial s}{\partial x^2}$
The metric is then said to be obtained by multiplying these basis vectors together in each of their possible combinations. In this case there are two basis vectors so there will be four elements in the metric:
$e_1e_1=g_{11}$
$e_1e_2=g_{12}$
$e_2e_1=g_{21}$
$e_2e_2=g_{22}$
$\implies g=\begin{bmatrix} g_{11} &g_{12} \\ g_{21}& g_{22} \end{bmatrix} = \begin{bmatrix} (e_1)^2 & e_1e_2 \\ e_1e_2& (e_2)^2 \end{bmatrix}$
This much I follow. Now, as I understand it, the metric is a diagonal matrix. Take the Euclidian metric for eg.
$\begin{bmatrix} 1 & 0 \\ 0 & 1 \end{bmatrix}$
So in what way does $e_1e_2$ equal $0$?