Make sure OpenCensusTracer from tracing package can be correctly shut down (flushed)

[mgencur]

The problem is that currently the components that use SetupStaticPublishing or SetupDynamicPublishing can drop some Spans/Traces when being shut down or evicted (e.g. during upgrades).
The goal is to prevent dropping buffered traces before they're sent as indicated by upstream issue in OpenCensus.

The OpenCensusTracer has a Finish function that can be called to properly Close the OpenZipkin http reporter and flush buffered traces. However, the SetupStaticPublishing/SetupDynamicPublishing functions don't make the OpenCensusTracer available so that anyone can call the Finish function. Another problem is that the Finish function passes nil which triggers nil pointer exception when calling the Finish function.

I've verified that calling the Finish function on the returned OpenCensusTracer sends all remaining traces and makes them available in Zipkin backend. It is only required to pass &config.Config{} on this line instead of nil.

It would be good to enforce calling Finish before components are shutdown as long as there's no sleep time in that component by default before it is shut down or evicted (I think some components have the sleep time such as Knative Serving Activator).

Related discussion here and here

Expected Behavior

It is possible to flush traces on demand (possibly before shutting down a component).

Actual Behavior

Traces that were buffered before the component is shutdown are not properly sent to tracing backend.

Additional Info

[cardil]

This looks like super easy to fix.

I think we should force change the SetupStaticPublishing or SetupDynamicPublishing funcs, so they will return the (*tracing.OpenCensusTracer, error).

Such incompatible change will enforce all Knative components to align, and in the process fix this issue, by calling Finish on shutdown.

[github-actions]

This issue is stale because it has been open for 90 days with no
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[mgencur]

/reopen