[pkg/ottl] remove support from accessing top-level objects

[TylerHelmuth]

Description

Removes the path accessors for the shared contexts that allowed getting top level objects for:

• resource
• instrumentation_scope
• metric
• span

While these accessors existed the grammar didn't actually let you use them and their presence makes future work problematic. At this time we don't want to provide accessors to entire parent objects directly.

Link to tracking issue

Related to #36869

[edmocosta]

Although it's not a breaking change, I'm wondering if we should have a change log for that, so users with this kind of no op statements would know why their configurations started to fail. If you think that's not needed, LGTM!

[evan-bradley]

I think we need a changelog entry documenting this as a breaking change, but maybe I'm missing something. I was able to get the following statement to work:

set(cache["metric"], metric) where resource != nil

I can't come up with any real uses for this, but it does parse. Honestly, given that it's so unhelpful, I'm tempted to even say we could call this a bug fix.

[TylerHelmuth]

I was able to get the following statement to work

It works when you're reaching up, but not when you're in the context itself.

So

context: datapoint
statements:
  - set(cache["metric"], metric)

works but

context: metric
statements:
  - set(cache["metric"], metric)

does not. I am ok calling this a bug_fix or a breaking change.