OTEL metrics are gated on `analytics.enabled`, even for custom `otel.metrics_exporter` configurations

What issue are you seeing?

If analytics.enabled is false, a user's explicitly configured otel.metrics_exporter is overriden to None.

https://github.com/openai/codex/blob/main/codex-rs/core/src/otel_init.rs#L70-L77

What steps can reproduce the bug?

Set analytics.enabled to false, provide a custom otel.metrics_exporter, launch codex (e.g. the TUI).

What is the expected behavior?

I would expect analytics.enabled = false to disable exporting metrics to the default Statsig endpoint, but not gate metrics exporting to explicitly provided endpoints.

Additional information

_No response_