Add fetchpriority=high to IMG when it is the LCP element on desktop and mobile with other viewport groups empty

[ShyamGadde]

Summary

Fixes #1710

Relevant technical choices

• Updated OD_URL_Metric_Group_Collection::get_common_lcp_element() to also consider an LCP element as common if it is identical in the narrowest and widest viewport groups, with all intermediate groups remaining empty.

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Co-authored-by: ShyamGadde <shyamgadde@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: felixarntz <flixos90@git.wordpress.org>

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[westonruter]

This is looking great! Could you also update \Test_OD_URL_Metric_Group_Collection::test_get_common_lcp_element() to add a data provider to test each of the return conditions? I see right now it is lacking in that it only tests the case where it returns an OD_Element. It isn't testing the cases when it can return null.

[felixarntz]

Great work @ShyamGadde, LGTM!

[westonruter]

Test cases look great, although I believe they can be a bit simplified. No need to include OD_Element::class in the data provider since it's always going to be the same if the expected is not null.

[westonruter]

It is a gift! 🎁