product_labels/views/product_product.xml

<?xml version="1.0" encoding="UTF-8"?>
<openerp>
    <data>

        <record id="product_normal_form_view" model="ir.ui.view">
            <field name="name">ProductProductViewFormInherit</field>
            <field name="model">product.product</field>
            <field name="inherit_id" ref="product.product_normal_form_view" />
            <field eval="900" name="priority"/>
            <field name="arch" type="xml">
                <xpath expr="//sheet" position="before">
                    <header>
                        <button class="oe_highlight" type="object" string="Etiqueta" name="print_labels"/>
                    </header>
                </xpath>
            </field>
        </record>

    </data>
</openerp>