Programmatically get hierarchy depth of a model (subsystems and referenced models)

I can get the hierarchy depth of a model using subsystem depth metric from the Verification and Validation toolbox, but this metric does not follow (i.e. count) the referenced models.

Depth traversal analysis stops when it reaches a referenced model or a library. 
Depth and level is restarted with 0 for each of these components.

I do not understand why this metric has this behavior. Why doesn't it count the depth following the referenced models?

In order to achieve this (calculate hierarchy depth following both subsystem and referenced models) the most efficient way, can I create a custom metric extending the subsystem depth metric? In this case, I'll have to locate code for the subsystem depth metric; I don't know where to get this code. Or are there any existing programmatic API to get the metric I'm interested in?