[prometheus] Stabilize Prometheus -> OTLP: Metadata

[dashpole]

What are you trying to achieve?

Mark the conversion of Prometheus metadata (type, unit, help) to OTLP as stable.

Open Questions:

translation SHOULD provide configuration which, when enabled, removes type (e.g. _total) and unit (e.g. _seconds) suffixes.

Should this be May? Should we specify the name of this configuration option?