Semconv code generation prototype

[lmolkova]

This is a prototype (related to https://github.com/open-telemetry/semantic-conventions/issues/551) of new code-gen approach:

• generates stable attributes in io.opentelemetry.semconv
• generates experimental attributes in io.opentelemetry.semconv.vA_B_C
• groups attributes by root namespace and generates them in multiple files

The process of codegen might looks like:

• keep old Semantic|ResourceAttributes with all deprecated things, delete at some point in the future
• update the version
• generate a new folder for experimental, update stable

As a result, we only need to care about back-compat when a stable attribute changes in a breaking manner. Multiple instrumentation libs can send different semconv versions at once.

Bloating: is indeed a problem

• the size of the artifact produced with this change is 131 KB
• if I generate a prev version - v1.22.0, it becomes 202 KB

TODO:

• [x] Figure out how to populate schema URL
  • manually for now (with automated check)
  • not possible(?) with jinja, need some custom script to add new value to Java file
  • [ ] do the script
• [x] stable and deprecated are mutually exclusive now, so template would not work in the future if a stable attribute gets deprecated - https://github.com/open-telemetry/semantic-conventions/issues/582

[jack-berg]

Closing as this is superseded by #45.