OpenCensus metric bridge

[dashpole]

trafficstars

Problem Statement

After https://github.com/open-telemetry/opentelemetry-specification/pull/2951, and https://github.com/open-telemetry/opentelemetry-specification/pull/2979, we could implement a metric bridge for OpenCensus.

Note that the specifications are experimental, so it isn't critical that it is implemented at this time.

Changes:

• [ ] Define a MetricProducer interface, following https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#metricproducer
• [ ] Define a RegisterProducer(metricProducer) function on MetricReader, following https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#registerproducermetricproducer
• [ ] Create an OpenCensus metric bridge which implements the MetricProducer interface, following https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/compatibility/opencensus.md

cc @aabmass