title: What is the recommended way to find the (position/velocity) indices of a free body in the `MultibodyPlant` state vector?
body: <p>There are a number of use cases where we want to introspect the state vector coming out of the <code>MultibodyPlant</code> <code>get_state_output_port</code>.  As a simple example, imagine I am logging the state output port and just want to label my axes (or know which axes to plot).</p>
  <p>What is the recommended API for this?
  For joints, I know that we can iterate through joints (e.g. via <code>get_joints()</code>) and then ask each joint for it’s position/velocities indices.  Do we have something similar for free bodies?</p>
  <p>I know we can get the actual positions/velocities of a free body in a number of ways, given a context, but didn’t actually see the way to get the indices?</p>
  <p>It’s tempting to say “just log with <code>get_state_output_port(model_instance)</code>”, but this particular  <code>MultibodyPlant</code> is buried inside a <code>Diagram</code> (like <code>MultibodyPlant</code>).</p>