OpenCensus metric bridge

[dashpole]

Problem Statement

After open-telemetry/opentelemetry-specification#2951, and open-telemetry/opentelemetry-specification#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

[github-actions]

This issue is stale because it has been open 60 days with no activity. Remove stale label or comment or this will be closed in 14 days.

[aabmass]

Can someone please assign me? I'll also be working on the opencensus trace bridge.

[github-actions]

This issue is stale because it has been open 60 days with no activity. Remove stale label or comment or this will be closed in 14 days.

[github-actions]

This issue was closed because it has been stale for 14 days with no activity.

[aabmass]

First step is implementing MetricProducers #4006