[3.13] AI Gateway: Document OpenTelemetry tracing for AI spans

[tomek-labuk]

Description

In 3.13 we are adding OpenTelemetry instrumentation for AI spans using the GenAI semantic conventions. Update the docs to describe the new tracing behavior, list the relevant AI span attributes, and reference the OTel spec:

https://opentelemetry.io/docs/specs/semconv/registry/attributes/gen-ai/#genai-attributes

Supported metrics: https://docs.google.com/spreadsheets/d/1c8dP4Z3qpBbFpGXcnCb_J0BJWMfq5Axxtna2OLEQHeg/edit?gid=642067573#gid=642067573

DoD

• Added reference document about spans/metrics that we currently support. Note: it's all in the development phase as per the spec and we'll need a label for it. The section can be similar to what we we have for the current LLM metrics.

• Gateway Otel plugin will need some kind mention and a link to the above-mentioned document
• A how-to guide using AI Proxy / Otel plugin and Jaeger which shows how to collect different spans for LLM requests.

Information

Person of contact: @nowNick

Size

M to L