Convert gen_ai.operation.name to enum and use it on spans

[lmolkova]

Replaces #995

Changes

• Converts gen_ai.operation.name to enum. Clarifies name for the chat operation.
• Adds it to span attributes
• Uses it in span name
• adds minor clarifications for gen_ai.system

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.

[lmolkova]

/cc @open-telemetry/semconv-llm-approvers

[drewby]

Looks good.

[codefromthecrypt]

drive by feedback, the description of this PR, while significantly changing a fundamental field (span name) does cover "what" like mechanics, but not "why"

Can we here and in the future put the motivation for renovations such as these, especially when doing things like enums we expect to break later? It will help be able to jump into a PR and not just assess, yeah.. this changes the span name, but also understand why we are considering it will drift all instrumenters.

[karthikscale3]

Just wanted to add a note from Langtrace's point of view, regarding the convention for span names, it’s a bit tricky to make the change in our case for us since our SDK provides an option to disable tracing certain methods if the user wants more control and as a result we set span name as the method name. We will have to make some changes to the "disable tracing logic" if we update this - so treading a bit cautiously here. Will make this change pretty soon.