Clarify string type in namespace

[nmattia]

This helps older (1.93) rust versions infer the correct types and works around the following error:

expected &[&String], found &[&str; 2]

[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to c8222fa.

Security Overview

• 🔎 Scanned files: 2 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/2f4ca4b6-3e66-473c-8350-2a581fad9f23?scanId=ef03dee4-2f80-432f-a3c2-5fd1f7bf29da&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Refactor	► src/routing/ic/route_provider/health.rs     Update label values for metrics to use as_str() ► src/routing/ic/routing_table_manager.rs     Update label values for metrics to use as_str()