This implied that the distance is a distance of angles multiplied by a metric, but what does the metric represents? The length of the joint? Why is that then linear?
imagine a robot in configuration A, a robot in configuration B, and a robot in configuration C. We want to know which is shorter: move from A to B, or to move from A to C? In order to answer that question, we somehow need to be able to measure a distance between 2 configurations? do we simply square all joint angles, add them, then extract the square root? And what happens if we have linear and angular actuators? Is one radian equivalent to 1 meter (in terms of distance)? A distance metric allows to help answer above questions. The way the metric is defined depends on the task, the robot, etc.
Actually yes, but this really depends on your robot and application. In a serial manipulator for instance, you usually want to give more weight to the base joints, since a displacement of them can cause a large displacement of the end-effector.