Bug: OpenTelemetry Semantic Convention compliance

[pjirsa]

Describe the bug
The span attributes emitted for token counts does not align with property names defined in Semantic Conventions 1.30 spec.

Existing attributes:

• gen_ai.response.completion_tokens
• gen_ai.response.prompt_tokens

Compliant attributes: Generative AI Traces

• gen_ai.usage.output_tokens
• gen_ai.usage.input_tokens