During the debugging process of btree index, I find adding the number of key nodes to shape is useful, it gives me the quick overview of how many keys are added into each btree bucket.

This 'shape' interface, I suppose, is used mainly for debugging purpose, no? So changing the output a bit should be OK.

Using 'dump' interface to dump all data is nice to have, but sometimes, it pollutes the output and make debugging less happy. That's why I need this feature, less verbose than 'dump', but more info than current 'shape'.

A simple fix will be added.