Remove MetricProducer, and OpenCensus bridges wrap existing interfaces instead of using MetricProducer

[dashpole]

Closes #3599

Changes

Instead of graduating MetricProducer, we could remove it, and attempt to work around its absence. This proposes "wrapping" push exporters and metric reader.

This is loosely based on a prototype in Go: open-telemetry/opentelemetry-go#4378

Pros:

• No new API or SDK interfaces to achieve roughly the same behavior.

Cons:

• Two interface implementations required to build a bridge. At least they can share most of the logic.
• MetricReader and pull metric exporters are very loosely specified. I'm not sure this will work across languages.
• In Go, it required changes to the Prometheus exporter to allow it to accept a Reader to make this work. This would allow users to change the aggregation temporality of metrics sent to the prometheus exporter.

[dashpole]

Closing based on #3599 (comment)