[prometheus] Stabilize Prometheus -> OTLP: Instrumentation Scope

[dashpole]

What are you trying to achieve?

Stabilize https://opentelemetry.io/docs/specs/otel/compatibility/prometheus_and_openmetrics/#instrumentation-scope

I don't think the specification needs any changes, but we should make sure this is properly implemented in the Prometheus receiver before we stabilize the specification.