feat: Add OpenTelemetry publish sample

[mukund-ananthu]

Fixes #<issue_number_goes_here> 🦕

[snippet-bot]

Here is the summary of changes.

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• samples/snippets/publisher.py:189, tag pubsub_publish_otel_tracing

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[conventional-commit-lint-gcf]

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[hongalex]

Can you sample at 100% to begin with and add a comment to lower the trace in production?

I can imagine customers who use the sample don't see that there are any traces exported and think something is broken.

[mukund-ananthu]

I use 1/100 with 10k messages published. That ensures there would be atleast 100 samples exported.

[mukund-ananthu]

Done, changed it to 1 based on offline discussion.

[hongalex]

I think we should omit this, since it's not necessary to wrap the traces in this way.

[mukund-ananthu]

Done. This would mean, there would be no parent trace. But as discussed offline, we are good with this / this is intended.