Fix a regression in our metadata backwards compatibility

    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.