[codex] Declare Prometheus metric types

[winhowes]

Summary

• declare built-in AuthTranslator metric families as Prometheus counters and histograms in the exporter output
• assert the emitted metric type metadata in the existing metrics tests
• update the example metrics plugin and docs so custom counters emit matching # TYPE lines too

Why

Grafana was warning on rate(authtranslator_requests_total[5m]) because the exporter emitted raw samples without Prometheus type metadata. The built-in *_total series are monotonic counters, so the fix is to declare them as counters instead of leaving them untyped.

Validation

• go test ./app/metrics/...
• go test -tags example ./app/metrics/plugins/example

[winhowes]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Can't wait for the next one!

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