Semconv schema: formalize registry definition

[lmolkova]

Today we require all attribute definitions to happen in the registry, i.e. :

• attribute definitions should be in /model/registry
• the group id must start with registry.

We should consider having a more formal way to mark a group as one defining something.
E.g.

groups:
- id: foo.bar
  type: attribute_registry <-------- this is new

or

groups:
- id: new-guid
  type: metric_registry <-------- this is new

This would simplify filtering/massaging semconv in code-generation, we'd avoid relying on specific group naming.
This would move post-factum policies enforcing attribute definitions into schema validation (id can only appear on *_registry groups and ref cannot appear there).

[lmolkova]

It's a good question if we can reuse existing types (attribute_group, metric, etc) and I believe the answer is no:

• we use attribute_group a lot to define a common group of ref attributes that are reused between multiple spans or metrics. This helps to avoid writing repetitive notes and similar for different http/messaging/etc metrics. E.g.

  semantic-conventions/model/metrics/http.yaml

  Line 2 in 21b8f46

  - id: metric_attributes.http.server

• we have precedents when existing metrics are referenced with caveats (such as specific error.type and notes)