chore: avoid checking instance on each stream call (#529)

* chore: avoid checking instance on each stream call

* fixed indentation

* added check for unary call

* fixed type check

* fixed tests

* fixed coverage

* added exception to test class

* added comment to test

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

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Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
2 files changed