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