Fix metadata backwards compat to handle deep nesting.

This is a bit confusing, but the behavior in 2.x was:

* a group's `metadata` did not have it's computed
  keys directly available; they were exposed off of
  the `:example_group` subhash.
* The `:example_group` subhash had a `:example_group`
  key that exposed the parent group's metadata
  (both user metadata and computed keys).

Before this, we were inserting extra `:example_group`
subhashes between the layers.