[prometheus] Stabilize Prometheus SDK exporter specification

[dashpole]

What are you trying to achieve?

This issue tracks the stabilization of the Prometheus exporter specification: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk_exporters/prometheus.md#metrics-exporter---prometheus

This is blocking the declarative configuration for the Prometheus exporter: open-telemetry/opentelemetry-configuration#467

This depends on the stabilization of the OTLP -> Prometheus translation spec: #4803

@open-telemetry/prometheus-interoperability