Add a flag in tracing config to use service instead of pod name

[pomyslowynick]

Add a flag in tracing config to use service instead of pod name

While the OTel migration is in progress it would be very helpful to have a flag which allows users toggling the ServingService to be used as the service.name in traces. This is related to this issue and would still be relevant after the migration to OTel.

From my own experience, service.name is used to group the traces together and when it's set to the pod name it's impossible to keep them all together.

This flag is a noop for existing configurations, since it defaults to false and will be used in another PR I've tested it in a live cluster for true/false/undefined values in the config map, in all the cases it worked as expected.

/kind enhancement

Release Note

You can now use the `useServingService` flag in the tracing config to use the `ServingService` as the `service.name` in traces. Current default is to use the `ServingPod` as the `service.name`, and it remains the same if the flag is not set.

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[knative-prow[bot]]

Welcome @pomyslowynick! It looks like this is your first PR to knative/pkg 🎉

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Hi @pomyslowynick. Thanks for your PR.

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[dprotaso]

/hold

I want to revisit this after the next release (Apr 22) - I think for the subsequent release we should do real work to migrate to OTel. Supposedly they are removing OpenCensus support in the OTel collector.

[dprotaso]

Closing this out - OTel work is in progress and I'll make sure that we use the 'right' service name in Serving.