URL Metric mutation helpers should be exposed in `initialize` args to match `finalize` args

[westonruter]

As discussed in #1928 (comment):

(Granted, this might be able to be refactored so that it happens at initialize(), so that in the handleLCPMetric() function it calls the extendRootData() function every time. This would depend on passing extendRootData() and extendElementData() to initialize() and we'd need to define urlMetric earlier. This would allow Image Prioritizer to get rid of its externalBackgroundImages variable since it would be directly putting the LCP data into the urlMetric whenever a new one is detected.)

This becomes more important when considering that if the browser is asked to do too much work during pagehide it may terminate execution of the function. See #1928. So the earlier we amend the URL Metric data the better.

These are the relevant properties of FinalizeArgs:

performance/plugins/optimization-detective/types.ts

Lines 77 to 83 in 47eef00

	readonly getRootData: () => URLMetric;
	readonly extendRootData: ( properties: ExtendedRootData ) => void;
	readonly getElementData: ( xpath: string ) => ElementData | null;
	readonly extendElementData: (
	xpath: string,
	properties: ExtendedElementData
	) => void;

We need to include them in InitializeArgs.

Then we'll need to move the client extension initialization logic down to occur after:

performance/plugins/optimization-detective/detect.js

Lines 682 to 689 in 47eef00

	urlMetric = {
	url: currentUrl,
	viewport: {
	width: win.innerWidth,
	height: win.innerHeight,
	},
	elements: [],
	};

Aside:

Let's add logging for each extension that is loaded.

We should also remove this old log entry (cf. d2e266d):

performance/plugins/optimization-detective/detect.js

Line 680 in 47eef00

log( 'Detection is stopping.' );

Lastly, let's move this line up to a more logical spot to right after it is last used (see 4025e61, although it should be placed right after disconnectIntersectionObserver()):

performance/plugins/optimization-detective/detect.js

Lines 888 to 889 in 47eef00

	// Clean up.
	breadcrumbedElementsMap.clear();

Finally, we should deprecate finalize because we cannot reliably compress the URL Metric data at pagehide.

[westonruter]

Embed Optimizer can be updated to use extendElementData() from initialize() as well. Instead of adding the resize observer in from initialize and then using this to populate a loadedElementContentRects which then gets passed in extendElementData() in finalize():

performance/plugins/embed-optimizer/detect.js

Lines 51 to 89 in 47eef00

	/**
	* Finalizes extension.
	*
	* @type {FinalizeCallback}
	* @param {FinalizeArgs} args Args.
	*/
	export async function finalize( {
	log: _log,
	error: _error,
	getElementData,
	extendElementData,
	} ) {
	/* eslint-disable no-console */
	// TODO: Remove once Optimization Detective likely updated, or when strict version requirement added in od_init action.
	const log = _log || console.log;
	const error = _error || console.error;
	/* eslint-enable no-console */
	for ( const [ xpath, domRect ] of loadedElementContentRects.entries() ) {
	try {
	extendElementData( xpath, {
	resizedBoundingClientRect: domRect,
	} );
	const elementData = getElementData( xpath );
	log(
	`boundingClientRect for ${ xpath } resized:`,
	elementData.boundingClientRect,
	'=>',
	domRect
	);
	} catch ( err ) {
	error(
	`Failed to extend element data for ${ xpath } with resizedBoundingClientRect:`,
	domRect,
	err
	);
	}
	}
	}

We can instead just call extendElementData() in the resize observer directly in the callback. This will eliminate additional work done during pagehide.

[westonruter]

As noted in #1893 (comment), I think we should consider eliminating the finalize API from extensions. It is somewhat dangerous because it could add delays or compute that causes execution of the pagehide event handler to be interrupted.