[SCHEMA] The MetricExporter schema is unsafe

[marcalff]

Currently:

• there is a unique schema for MetricExporter, which can represent otlp, console, prometheus, or an extension point.
• a PeriodicMetricReader uses a MetricExporter
• a PullMetricReader uses a MetricExporter

This is unsafe, because push and pull exporters are mixed in a common schema, which makes it possible to represent:

meter_provider:
  readers:
    - periodic:
        exporter:
          prometheus:
    - pull:
        exporter:
          console:

This is invalid:

• a periodic exporter is not going to use a prometheus scraper,
• a pull exporter is now going to use the console to scrape.

Please define instead two separate schemas:

• PushMetricExporter, which can represent otlp, console, or an extension point,
• PullMetricExporter, which can represent prometheus, or an extension point,

This also imply that a extension points for PushMetricExporter and PullMetricExporter are different, because they have a different type, which is good.

The component registry will need to expose both types.

[jack-berg]

Good call. Would you be willing to open a PR to fix?