Improve debugging stored URL Metrics in Optimization Detective

[westonruter]

While all of the URL Metrics are dumped out as part of the collection, it makes it difficult to drill down to see what the most recently-stored URL Metrics are. This is improved here, including a creationDate property which includes a Date instance of the timestamp:

[github-actions]

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Co-authored-by: westonruter <westonruter@git.wordpress.org>
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[westonruter]

@felixarntz Does this look good to you?

[adamsilverstein]

Nifty!