[prometheus] Stabilize Prometheus -> OTLP: Counters and Gauges

[dashpole]

What are you trying to achieve?

Stabilize https://opentelemetry.io/docs/specs/otel/compatibility/prometheus_and_openmetrics/#counters and https://opentelemetry.io/docs/specs/otel/compatibility/prometheus_and_openmetrics/#gauges

I don't think there are any open questions.