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
• expfmt: Implement OpenMetrics generation #214
• expfmt: Decode support OpenMetrics Text exemplars. #703