Migrate from OpenCensus to OpenTelemetry

[anniefu]

OpenCensus & OpenTracing have merged to form OpenTelemetry (see https://github.com/census-instrumentation/opencensus-go & https://medium.com/opentracing/a-roadmap-to-convergence-b074e5815289).

Knative should to move to instrumenting metrics and traces with OpenTelemetry instead of OpenCensus ahead of OpenCensus getting deprecated.

[grantr]

Looks like OpenTelemetry Go SDK is in beta now: https://github.com/open-telemetry/opentelemetry-go

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.

[vagababov]

/lifecycle frozen
/assign @evankanderson @jjzeng-seattle

[skonto]

I have done an evaluation on this topic here are the takeaways. From my POV otel is not ready yet, I would wait for GA. There is also a spike for this in wip status. We discussed this on a community meeting as well.

[evankanderson]

/unassign