Generalize each (single) Istio instance into two OpenMetricsBaseCheck instances

def create_generic_instances(self, instances):
        """
        Generalize each (single) Istio instance into two OpenMetricsBaseCheck instances
        """
        generic_instances = []

        for instance in instances:
            istio_mesh_instance = self._create_istio_mesh_instance(instance)
            process_mixer_instance = self._create_process_mixer_instance(instance)

            generic_instances.extend([istio_mesh_instance, process_mixer_instance])

        return generic_instances