Add opencensus compatibility spec compliance matrix

[dashpole]

Document the current state of OpenCensus bridges in the spec compliance matrix.

Go

• metric: https://github.com/open-telemetry/opentelemetry-go/blob/main/bridge/opencensus/metric.go#L40
• trace: https://github.com/open-telemetry/opentelemetry-go/blob/main/bridge/opencensus/bridge.go#L29

Java:

• metric: https://github.com/open-telemetry/opentelemetry-java/tree/main/opencensus-shim#metrics
• trace: https://github.com/open-telemetry/opentelemetry-java/tree/main/opencensus-shim#traces

Python

• trace: https://github.com/open-telemetry/opentelemetry-python/tree/main/shim/opentelemetry-opencensus-shim/src/opentelemetry/shim/opencensus

JS:

• trace: https://github.com/open-telemetry/opentelemetry-js/tree/main/experimental/packages/shim-opencensus