OpenMetrics is the standardization of prometheus's data export format, also adding (back) support for protobuf. It is currently a Standards Track Internet Draft (future RFC).
The specification reads like most RFCs. The main takeaways should be the naming conventions especially around data / unit types, everything else is handled by the libraries you'd usually use.
About the standard itself,
the data produced is a (standalone) snapshot,
which includes all the extra text (TYPE, UNIT, HELP)
which is nice for debugging,
but seems like quite a bit of overhead.