Tags should be inherited when extending semconvs

[lmolkova]

see https://github.com/open-telemetry/semantic-conventions/pull/698/files#r1525566689

When I do

- id: semconv
  ...
  attributes:
    - id: foo.bar
      ...
      tag: tech-specific

- id: child_senconv
  ...
  extends: semconv
  attributes:
    - ref: foo.bar

I expect to inherit tags from semconv., but it does not happen today.

I should still be able to override or remove them in the child_semconv.