[prometheus] Stabilize Prometheus -> OTLP specification

[dashpole]

What are you trying to achieve?

The Prometheus receiver is one of the components included in the OpenTelemetry Collector 1.0 scope. It depends on Prometheus -> OTLP portion of the compatibility specification: https://opentelemetry.io/docs/specs/otel/compatibility/prometheus_and_openmetrics/#prometheus-metric-points-to-otlp.

This issue tracks stabilizing that portion of the compatibility specification.

The following portions of the specification will remain experimental:

• Exemplars (which are still experimental in Prometheus).
• Info and StateSet - typed metric handling.

@open-telemetry/prometheus-interoperability