Add system metrics to moshi-server

[soyouzpanda]

I, soyouzpanda, confirm that I have read and understood the terms of the CLA of Kyutai-labs, as outlined in the repository's CONTRIBUTING.md, and I agree to be bound by these terms.

Checklist

• Read CONTRIBUTING.md, and accept the CLA by including the provided snippet. We will not accept PR without this.
• Run pre-commit hook.
• If you changed Rust code, run cargo check, cargo clippy, cargo test.

PR Description

Adding some system metrics to the /metrics endpoint:

• GPU Utilization
• GPU Memory utilization
• GPU Temperature
• CPU utilization
• System memory utilization

This allows better load balacing in a mutli-node environment