decode: Implement full OpenMetrics parsing support

[ywwg]

The common parser does not correctly decode open metrics text format. Prometheus' parser is highly optimized and specialized for its scraping use-case, and there is a need for a better externally-usable parsing/formatting library.

TODO:

• [ ] Support "unknown" keyword instead of prom's "untyped"
• [ ] Native histograms
• [ ] Support all quoting characters
• [ ] .... other things?

See also:

• [ ] https://cloud-native.slack.com/archives/C01AUBA4PFE/p1755087084043779?thread_ts=1753991269.132859&cid=C01AUBA4PFE
• [ ] https://github.com/prometheus/common/issues/214
• [ ] https://github.com/prometheus/common/issues/703