metrics: parse Prometheus text exposition with best practices

[naoNao89]

• Prefer parsed sample timestamps over fallback
• Preserve HELP as description; keep unsupported families as errors (for now)
• Remove gauge normalization in tests; only ensure trailing newline
• Pin openmetrics-parser to 0.4.4 (latest)

Refs: https://github.com/torrust/torrust-tracker/issues/1582

[da2ce7]

@naoNao89 this looks much better and cleaner :)