Port opencensus integration to opentelemetry

[slinkydeveloper]

Problem
Now we use opencensus, which is going to be deprecated, we need to switch to https://github.com/open-telemetry/opentelemetry-go

Persona:
Knative contributor

Exit Criteria
We don't depend anymore on opencensus

Time Estimate (optional):
🤷‍♂️

Additional context (optional)
We also need to handle this: knative/eventing-contrib#1155 (comment)

[lionelvillard]

Related to knative/pkg#855

[slinkydeveloper]

Oh thanks for spotting it!

[grantr]

Looks like OpenTelemetry Go SDK is in beta now: https://github.com/open-telemetry/opentelemetry-go. So the next step is making sure that OT meets all our requirements.

[slinkydeveloper]

@grantr do we have some document with these requirements defined? How would you proceed?

[vaikas]

@evankanderson @anniefu would you mind sharing what you had in mind? I know you had done a lot of work here.

[anniefu]

The upcoming metrics plan that Evan wrote up https://github.com/knative/pkg/blob/master/metrics/README.md is primarily about moving to a centralized collector service model for metrics.

Since, the OpenCensus Collector Service agent and the OpenTelemetry Collector Service agent will both support the OpenCensus client libraries, we can port from opencensus client libraries to the opentelemetry client libraries independently of the new metrics plan.

The last time we talked to OpenTelemetry back in february, their metrics models were still in flux and they recommended @evankanderson make changes to opencensus library to support Knative rather than trying to early-migrate to Otel client libraries.

Given that, I think we want the metrics plan to land and maybe wait beyond Otel beta before considering porting Knative libraries.

[anniefu]

In terms of knative/eventing-contrib#1155, tracing and metrics in Knative are handled completely separately right now, so it is possible to migrate to OpenTelemetry for tracing, but not metrics.

I'm not very familiar with the world of tracing, so I'll leave that judgment call to others.

[slinkydeveloper]

Links to cloudevents/sdk-go#554