chore(tests): Add cloud ops data tests

[daniel-sanche]

We recently had an error where no logs, metrics, or other cloud ops data was making it's way into sandbox's Cloud Operations dashboards, making the demo useless. This case was missed by our existing tests

• This PR attempts to fix this by rewriting monitoring_integration_test.py with tests that read from live observability data, to make sure it is available
• Note that these tests alone would not fix the issue, because our CI test project will always have the proper permissions set up. To help us test live sandbox environments easier, I also added sandboxctl test. This will let us run our test suite on fresh environments to make sure everything looks as it should, just like the CI tests do
• I also added a note to RELEASING.md, so we remember to check this case in our release process checklist

Related issue: https://github.com/GoogleCloudPlatform/cloud-ops-sandbox/issues/838

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