Add instructions to CONTRIBUTING for changing the agent log level #14104

Job	Run time
changed_files	7s
check_if_release_branch	1s
towncrier_check	8s
build mirrord CLI	0s
lint	12m 7s
check-rust-docs	4m 17s
lint_markdown	0s
integration_tests	12m 38s
macos_tests	24m 52s
test_agent	3m 20s
test_agent_image	6m 47s
test_cli_image	15m 31s
e2e (docker)	15m 9s
vscode_e2e_on_release_branch	0s
intellij_e2e_on_release_branch	0s
e2e (containerd)	12m 35s
ci	1s
	1h 47m 33s

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