[FIX] point_of_sale: pos order -> count of order

In the pivot view of report.pos.order that is a view mainly over
pos.order, the "Count" measure is a little rubish giving more or less
the number of different products in the grouped order lines.

With this changeset, we have the field "Order" as measure that allow us
to know the number of orders a grouping is corresponding to.

opw-2127275
closes #40723

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>

addons/point_of_sale/views/pos_order_report_view.xml

@@ -7,6 +7,7 @@
                 <pivot string="Point of Sale Analysis" disable_linking="True">
                     <field name="product_categ_id" type="row"/>
                     <field name="date" interval="month" type="col"/>
+                    <field name="order_id" type="measure"/>
                     <field name="product_qty" type="measure"/>
                     <field name="price_total" type="measure"/>
                 </pivot>