Update Prometheus Exporter with latest Metrics SDK

[legendecas]

After the major refactoring of Metrics SDK, we should update the Prometheus exporter to adopt the changes.

• Refactor metrics export data model to match proto #2775
• New Feature: Prometheus Compatbility Spec https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/datamodel.md#prometheus-compatibility
• Name Conversions (Define OTLP -> Prometheus metric name conversion rule opentelemetry-specification#2437)

[dyladan]

This may depend on #2775

[legendecas]

I'm working on this to check if there is anything to do to finalize the first alpha version of metrics SDK.

[dyladan]

Can this be closed since it works with the latest SDK? The resource support is a separate issue and is already tracked in #867

[legendecas]

Yeah, closing as completed.