Fixes #1950

Changes

Refines the values of gen_ai.system for Google's AI products.

The Google Gen AI SDK defined here:

• https://github.com/googleapis/python-genai

... provides a common client for both the Vertex AI and Gemini APIs and reflects some degree of intended sharing / convergence between these two systems. Having totally separate names (like vertex_ai and gemini) without a common prefix means that any vendor-specific attributes would need to be defined duplicatively for both vertex_ai and for gemini, even if supported by both systems.

Additionally, code built on top of the Google Gen AI SDK may not necessarily know which backend is being used.

This change proposes a common prefix (gcp.gen_ai) which may be used for any Google-specific Gen AI attributes that are common between both backend systems. More specific sub-systems (gcp.gen_ai.vertex_ai and gcp.gen_ai.gemini) still exist to allow for divergent properties where desireable. However, the common prefix should make it easier to provide unified Semantic Conventions where it makes sense to do so, simplifying conventions and instrumentation.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• [?] schema-next.yaml updated with changes to existing conventions.
  • Is it possible to rename attribute values this way? I see that attribute keys are renamable, but it's not clear whether it is possible to change the values this way.