doc: Subsribe sample

[mukund-ananthu]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

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Fixes # 🦕

[snippet-bot[bot]]

Here is the summary of changes.

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- [samples/snippets/subscriber.py:89](https://github.com/googleapis/python-pubsub/blob/8ff21704dc1373ee792dc80f174fda4855a873f8/samples/snippets/subscriber.py#L89), tag `pubsub_subscribe_otel_tracing`

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[conventional-commit-lint-gcf[bot]]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot https://conventionalcommits.org/

[mukund-ananthu]

@hongalex PTAL