Update Metrics SDK to latest Feature-Freeze Spec

[legendecas]

This issue tracks work needed to meet the SDK specification.

• [metrics SDK] Specification compliant MeterProvider #2593
• Implement Views API #2592
• Implement MetricReader #2590
• [metrics sdk]: verify and enforce attribute limits #2591
• Define MetricExporter interfaces #2589
• Metric Aggregations #2599
• AggregationTemporality Controls #2864
• Consolidate Observer and Counter example #868
• Multi-instrument observable callback #2956
• Default Instrument unit does not comply with spec #2910
• Asynchronous instrument gaps and resets on delta temporality  #2987

Prototype on exporters:

• Update Prometheus Exporter with latest Metrics SDK #2773
• Update OTLP Metrics Exporter with latest Metrics SDK #2774

After GA:

• Metrics examplars #2594

[legendecas]

Tried to sum up the internal design of the SDK: https://docs.google.com/document/d/1EluDhj5UanNATS3hR8xx30sv55Se7uYjsGYslvNWHtc/edit#.

Please feel free to comment :).

[vmarchaud]

Overall i'm not familiar with all the new mechanisms of the sdk (specially metrics stream) so i won't help much providing feedback on the implementation but i've made few comments on the wording. I'm hoping to be able to re-visit it when PRs are made to help a little bit more on reviews

[legendecas]

Closed and keep tracking in https://github.com/open-telemetry/opentelemetry-js/milestone/15.