fix: fully sanitize Prometheus label names (#267)

Prometheus label names must match `[a-zA-Z_][a-zA-Z0-9_]*`, but dynamic GPU detail keys previously replaced only spaces and hyphens. Keys such as `Source: Fan` therefore produced invalid exposition and could make scrapes fail.

Replace every character outside the documented ASCII grammar, lowercase valid ASCII letters, prefix names that begin with a digit, and map empty names to `_`. Cover the sanitizer invariant and the real GPU info exporter path with regressions.