Prevent rendering Key Metrics if setup not completed

## Bug Description

@aaemnnosttv: This isn't a particularly realistic scenario.

## Steps to reproduce

1. Setup and enable the KMW widget.
2. In the database, delete the `googlesitekit_key_metrics_setup_completed_by`
3. Refresh the dashboard and the key metrics still render alongside the Key metrs Setup CTA Banner.

## Screenshots


## Additional Context

- PHP Version:
- OS: [e.g. iOS]
- Browser: [e.g. chrome, safari]
- Plugin Version: [e.g. 22]
- Device: [e.g. iPhone6]

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## Acceptance criteria

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## Implementation Brief

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### Test Coverage

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## QA Brief

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## Changelog entry

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