[codex] Add latency phase metrics

[winhowes]

What changed

• renamed the old upstream latency metric to authtranslator_upstream_roundtrip_duration_seconds
• added authtranslator_end_to_end_duration_seconds
• added authtranslator_pre_proxy_duration_seconds
• added authtranslator_response_processing_duration_seconds
• updated the duration recorder, Prometheus export, tests, and observability docs to match the new metric model

Why

The old authtranslator_request_duration_seconds name implied total request latency, but the implementation only measured proxy handoff until an upstream response was received. That made it hard to reason about total client-visible latency versus latency AuthTranslator itself introduces.

Impact

• dashboards and alerts using authtranslator_request_duration_seconds need to migrate to the new metric names
• callers can now break latency down into request-side AuthTranslator overhead, upstream wait time, response-side AuthTranslator processing, and full end-to-end latency

Root cause

The existing metric name was broader than the timing window it actually captured, and the proxy did not emit separate timings for pre-proxy or response-side processing.

Validation

• go test ./...

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 6c35e7a997

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[winhowes]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Nice work!

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[winhowes]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Swish!

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[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 👍

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

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