Keep renamed/removed experimental attributes as deprecated

[lmolkova]

When renaming/removing an attribute, we need to preserve the old name in the yaml.

Currently it disappears, meaning that:

• codegen does not generate it from yaml
• semconv lib maintainers need to manually copy them back to avoid breaking API changes
  • This creates a lot of work for language SIGs
    • Develop strategy for generating semantic conventions in different signals, different domains semantic-conventions-java#5
    • or here's how python list of manually defined deprecated attributes looks like
• OR semconv libraries drop removed attributes from API surface
  • applications that don't have all the versions of instrumentation libraries perfectly aligned can fail in runtime because of missing attributes in newer versions.

Options:

1. Never remove an attribute, when renaming always add the old one to deprecated ones
   • we'd also need CI checks to validate
2. Add build-tools feature to compare list of attributes between semconv vOld and vNew and generate list of removed attributes.

[lmolkova]

My preference would be on Option 2 since we need to build validation tooling anyway, so why not automate the whole process.

[AlexanderWert]

@lmolkova For HTTP we have (at least for some of them) the deprecated attributes listed in a separate section in the registry: https://opentelemetry.io/docs/specs/semconv/attributes-registry/http/#deprecated-http-attributes

I like to have this documented not only in yaml files (and codegen) but also in the docs.

So how about adopting the same for all attributes that are being removed / renamed / deprecated?

[lmolkova]

@AlexanderWert we've been inconsistent with marking them as deprecated.
So either we embrace it and pick option2 or add tooling to ensure we mark removed ones as deprecated (option1).

I agree that having them preserved in the spec (at least for a certain period of time) is great.

[lmolkova]

closing in favor of #740