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

[krinkinmu]

@ywwg by any chance was there any progress on this?

We want to use the common parser in Istio, but given that it does not support OM we can't and have to work around it.
I'd like to get an idea if there is (or was) any ongoing work to address it behind the scene that isn't visible in the bug?