Gate test-only worker telemetry helper

[edo-chan]

Summary

• gate WorkerTelemetry::from_meter behind the same test-utilities / test cfg used by the helper paths that call it
• remove the dead-code warning from normal builds without changing runtime behavior

Testing

• cargo +nightly fmt
• cargo lint
• cargo test
• cargo integ-test

[claude]

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[Sushisource]

Thanks! I kept meaning to get around to this.