test(spanner): revert noxfile changes and disable AFE server timing in conftest

[sakthivelmanii]

Reverts the edits to packages/google-cloud-spanner/noxfile.py from PR #17970 to restore compliance with owlbot/librarian autogeneration and fix issue #17974.

Disables AFE server timing in unit tests directly inside tests/unit/conftest.py by setting SPANNER_DISABLE_AFE_SERVER_TIMING="true" and _helpers.ENABLE_AFE_SERVER_TIMING = False. This ensures both standard unit tests and prerelease-deps unit tests pass cleanly without modifying autogenerated files or existing unit test assertions.

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:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request centralizes the disabling of AFE server timing for unit tests by moving the configuration from noxfile.py environment variables to tests/unit/conftest.py. The reviewer suggests using monkeypatch.setattr instead of manually setting and resetting _helpers.ENABLE_AFE_SERVER_TIMING to leverage pytest's built-in teardown mechanism and prevent state leakage.

[gemini-code-assist]

Instead of manually setting and resetting _helpers.ENABLE_AFE_SERVER_TIMING directly, use monkeypatch.setattr to leverage pytest's built-in teardown mechanism. This is more idiomatic and guarantees that the original state is restored automatically after each test, preventing potential state leakage.

Suggested change

	monkeypatch.setenv("SPANNER_DISABLE_AFE_SERVER_TIMING", "true")
	_helpers.ENABLE_AFE_SERVER_TIMING = False
	SpannerMetricsTracerFactory._metrics_tracer_factory = None
	SpannerMetricsTracerFactory._current_metrics_tracer_ctx.set(None)
	yield
	# Reset singletons after test to ensure no leakage
	_helpers.ENABLE_AFE_SERVER_TIMING = False
	SpannerMetricsTracerFactory._metrics_tracer_factory = None
	SpannerMetricsTracerFactory._current_metrics_tracer_ctx.set(None)
	monkeypatch.setenv("SPANNER_DISABLE_AFE_SERVER_TIMING", "true")
	monkeypatch.setattr(_helpers, "ENABLE_AFE_SERVER_TIMING", False)
	SpannerMetricsTracerFactory._metrics_tracer_factory = None
	SpannerMetricsTracerFactory._current_metrics_tracer_ctx.set(None)
	yield
	# Reset singletons after test to ensure no leakage
	SpannerMetricsTracerFactory._metrics_tracer_factory = None
	SpannerMetricsTracerFactory._current_metrics_tracer_ctx.set(None)