Add Arize AX collector guidance to OpenTelemetry docs

[arizedatngo]

Summary

• Document how to forward Open WebUI's built-in OpenTelemetry traces through a collector when an OTLP backend requires custom authentication headers.
• Use Arize AX as a concrete example because it requires space_id and api_key exporter headers that Open WebUI cannot configure directly.
• Clarify that this path exports backend service telemetry; rich prompt, response, token, and tool-call traces require separate instrumentation on the model-provider or Pipeline path.

This benefits self-hosted Open WebUI operators who already use the documented OTLP export but cannot connect directly to a backend requiring non-basic-auth headers. The collector pattern is reusable for other OTLP backends with custom-header requirements.

Related issue or discussion

No linked issue. This extends the existing Custom Collector Setup and Authentication required? sections of the OpenTelemetry reference.

Checklist

• I have reviewed the relevant documentation and matched the existing style.
• This PR meets Open WebUI's contribution standards: it is accurate, relevant to users, narrowly scoped, maintainable, and not promotional content, advertising, lead generation, SEO placement, or a request to list a product, service, provider, integration, gateway, tool, or company primarily for visibility.
• I understand that PRs that do not meet these standards may be closed without review and will not be merged. Repeated, low-quality, off-topic, promotional, or intentionally misleading submissions may result in the contributor being blocked from future participation in Open WebUI repositories.

Notes for reviewers

• Rebased through a normal merge with current main; the content change remains confined to docs/reference/monitoring/otel.md.
• The collector configuration matches Open WebUI's current OTLP/gRPC setup and uses an otlphttp exporter to add backend-specific headers.
• npx prettier@3.3.3 docs/reference/monitoring/otel.md --check passes.
• git diff --check passes.

[arizedatngo]

@silentoplayz could you help clarify the contribution route for this closed documentation PR? I saw you merged the latest docs changes (#1386 and #1383). #1365 was closed without an explanatory comment in the collected history. Is there an appropriate page or accepted scope for Arize AX/Phoenix OpenInference guidance? I would like to adapt to the docs team’s requirements before proposing a replacement.

[silentoplayz]

@silentoplayz could you help clarify the contribution route for this closed documentation PR? I saw you merged the latest docs changes (#1386 and #1383). #1365 was closed without an explanatory comment in the collected history. Is there an appropriate page or accepted scope for Arize AX/Phoenix OpenInference guidance? I would like to adapt to the docs team’s requirements before proposing a replacement.

Following the Pull Request template is a strict requirement. I didn't close your PR by the way. @Classic298 did.

[arizedatngo]

@Classic298 @silentoplayz thanks for clarifying the template requirement. I have kept the work on this original PR, updated its body to the current required template/checklist, merged current main into the branch, and clarified in the docs that this collector path exports Open WebUI's built-in backend telemetry rather than full prompt/response/token/tool traces. The change addresses the concrete limitation that Open WebUI supports OTLP endpoint and basic-auth settings but cannot directly configure the custom exporter headers some OTLP backends require; the collector pattern itself is reusable. Prettier and git diff --check pass. GitHub would not let me reopen the closed PR as the author—could you reopen it for review if this is now in the acceptable scope, or identify the remaining requirement on this same PR?