We patch OpenCensus to add SetSpanID method

[grantr]

#1972 added a patch to the vendored opencensus library that adds the SetSpanID method.
@Harwayne calls this a viral patch: any project importing eventing will also need to patch their opencensus.

This was used only as a last resort because OpenCensus doesn't provide the functionality we need to set the parent span id (see pkg/tracing/traceparent.go). We don't expect OpenCensus to fix this issue, but eventually we'll switch to OpenTelemetry which will hopefully make the patch unnecessary.

Everywhere this patch is used, we should add a comment that links to this bug.

Expected behavior
We never patch vendored code.

To Reproduce

1. Try importing knative.dev/eventing/pkg/tracing, or knative.dev/eventing and one of the eventing commands that supports tracing.
2. Observe the build fails with vendor/knative.dev/eventing/pkg/tracing/traceparent.go:83:6: span.SetSpanID undefined (type *trace.Span has no field or method SetSpanID)

Knative release version
0.10.0

[mattmoor]

We don't expect OpenCensus to fix this issue

Have we asked them? Have we opened a PR to add this functionality upstream?

This was a bit of a shock for me because generally when we do this we reference the version we're backporting it from (e.g. higher kube version) or the PR where we're adding it upstream, but there are no discernable attempts to do so here.

[grantr]

I believe there was certainty that this change would not be allowed, but I don't remember the details and haven't been able to find them after some digging. It might be worth contacting OpenCensus again to get some fresh eyes.

[grantr]

I asked around and it seems this change was never floated to the OpenCensus community. So

Have we asked them? Have we opened a PR to add this functionality upstream?

No, and no, but should and should :)

[mattmoor]

@grantr thanks and thanks :)

[ian-mi]

It appears that this patch is needed to adopt the parent span without creating a new child span. Have we considered whether the correct behavior is actually to create a new child span in the channel dispatcher? I can see this span being potentially useful.