Ensure URL Metric is initially constructed with all elements prior to initializing extensions

[westonruter]

Summary

This is a follow-up to #1951.

When testing the plugin I noticed an error in the console:

Optimization Detective] Failed to initialize extension 'http://localhost:8888/wp-content/plugins/od-intrinsic-dimensions/detect.js?ver=0.2.0': Error: Unknown element with XPath: /HTML/BODY/DIV[@class='wp-site-blocks']/*[2][self::MAIN]/*[1][self::DIV]/*[3][self::DIV]/*[1][self::FIGURE]/*[1][self::VIDEO]
    at extendElementData (detect.js?ver=1.0.0-beta4:403:9)
    at captureIntrinsicDimensions (detect.js?ver=0.2.0:34:3)
    at Module.initialize (detect.js?ver=0.2.0:72:4)
    at detect (detect.js?ver=1.0.0-beta4:821:41)

The error coming from the Intrinsic Dimensions extension started occurring after westonruter/od-intrinsic-dimensions#4 where I updated it to use initialize instead of finalize since it was deprecated in #1951 in order to facilitate gzip compression in #1959. However, I realized that the location where extensions are initialized means that they cannot call extendElementData() because the URL Metric object has not been populated yet with its elements. This causes calls to extendElementData() to always fail if they are performed while the initialize() function is being called (and not after a delay, for example).

The fix is simply to move the extension initialization logic after the URL Metric data has been initially constructed with all of the tags (elements) being tracked.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: westonruter <[email protected]>
Co-authored-by: ShyamGadde <[email protected]>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[ShyamGadde]

Makes sense—thanks for catching this! LGTM.