[Prometheus] Exporter should support 'NoTranslation' mode

[ywwg]

Problem Statement

Currently the only way to control escaping in the prometheusexporter is to set common/model.NameValidationScheme. This value is deprecated and shouldn't be used this way.

Proposed Solution

We should unify the configuration options with the consensus solutions being implemented in Prometheus and Otel-Collector: Supporting an explicit "NoTranslation" mode that exposes unescaped metrics on the Prometheus endpoint.

The consensus solution involves two booleans and three possible settings:

1. "no-translation" mode -- native Otel metric and label names
2. Prometheus-translation mode with suffixes
3. Prometheus-translation mode without suffixes

The current consensus is not to support NoTranslation + EnableSuffixes. If there is demand for this mode, for instance to prevent metric name collisions, we can add it.

Alternatives

Other methods could be used to control the chosen escaping scheme, like a library global variable, but that's a poor way to control behavior.

Prior Art

OtelCollectorConbtrib's prometheusexporter and prometheusremotewriteexporter are both moving towards config values. Prometheus' otel endpoint similarly will have config-controlled settings.

[ywwg]

Also, can a codeowner please assign me to this bug

[dmathieu]

Related: #6590

You have more context than me here, but I wonder if we need that configuration at all.
In a not-too-far future, this config will only have one possible value, yet we will have to support it until we have a major release.

I know relying on deprecated configs isn't great. When we discussed about that at Kubecon, @bwplotka was considering removing the deprecation notice.

[ywwg]

In a not-too-far future, this config will only have one possible value

I think I drafted this issue badly, conflating the Escaping Scheme configuration with the availability of configuration to determine how metrics can be translated for scraping by prometheus.

See: open-telemetry/opentelemetry-collector-contrib#35459

Currently the predicted end state is three options in the SDK:

1. "no-translation" mode -- native Otel metric and label names
2. Prometheus-translation mode with suffixes
3. Prometheus-translation mode without suffixes

We currently have AddMetricSuffixes already, we just need to add TranslationStrategy similar to prometheus/prometheus#16441. We understand this will require updating the spec per open-telemetry/opentelemetry-collector-contrib#39706.

This is also related to: #6704

I will update the wording of this bug to better reflect the intent.