chore(main): release stackdriver-core 1.8.0 #106155

Job	Run time
CI (ubuntu-latest, 4.0, --rubocop-toplevel --rubocop --build --yard --linkinator)	58s
CI (ubuntu-latest, 3.4, --test)	53s
CI (ubuntu-latest, 3.3, --test)	1m 2s
CI (ubuntu-latest, 4.0, --test)	46s
CI (ubuntu-latest, 3.1, --test)	45s
CI (ubuntu-latest, 3.2, --test)	54s
CI (macos-latest, 4.0, --test)	1m 3s
CI (windows-latest, 4.0, --test)	2m 5s
	8m 26s

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