Add Arize AX to OpenTelemetry examples

[arizedatngo]

Summary

• Add Arize AX to the list of OTLP-compatible backends in the voice agent OpenTelemetry example.
• Include the Arize AX OTLP/HTTP endpoint, required headers, and project metadata example.
• Update the agent observability knowledge-base text to name multiple OTLP-compatible backends.

Notes

This is a docs/example-only update. The existing example already supports arbitrary OTLP/HTTP endpoints and headers through setup_otel; this adds provider-specific guidance for users sending LiveKit agent traces to Arize AX.

Testing

• git diff --check

[CLAassistant]

All committers have signed the CLA.

[arizedatngo]

Addressed the Devin review item by setting openinference.project.name as an OpenTelemetry resource attribute instead of LiveKit metadata. That keeps Arize project selection on the tracer provider/resource path, while session metadata remains reserved for LiveKit job/session attributes.

[arizedatngo]

@chenghao-mou hi, looks like you recently worked on the OpenTelemetry example and reviewed the homepage voice-agent example. If you are the right reviewer for this area, could you take a look when you have a chance? CLA, ruff, type-checks, unit tests, and Devin Review are passing now; the earlier Devin concern was addressed by moving the Arize project name to an OpenTelemetry resource attribute.