Label trace diagnostics as trace metrics

[liurenfeng94-ops]

This came up because the trace diagnostics looked too similar to the realtime metrics, so I made the trace ones say Trace check ms and marked the diagnostics source/code explicitly in VS Code. I also pulled the message formatting into a small helper and covered it with tests.

Validation:

• pnpm vitest run test/trace-diagnostic-message.test.ts
• pnpm typecheck
• pnpm exec eslint .
• pnpm build

[github-actions]

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[liurenfeng94-ops]

Thanks for the note — I rewrote the description to be more direct. The change itself is small: it makes the trace diagnostics explicitly read as trace metrics, and it tags the diagnostics so they are clearly separate from the realtime measurements. I can walk through the code path end to end if helpful.