Something that has bugged me for awhile with how tag visitor callbacks work is the aspect of returning a boolean for whether or not the currently-visited tag should be tracked in the elements of the stored URL Metrics. For simple cases, the boolean works fine but it can get hard to track when the callback returns in multiple places. If you have to short-circuit anywhere, then you have to remember which return value is appropriate at a given time.

The other problem with returning a boolean is having to remember what the boolean means. This was difficult to explain when reviews were performed when developing Optimization Detective, and it never totally felt right. Additionally, when developing a tag visitor which doesn't actually need URL Metrics to perform its optimizations, it is confusing that in this case there is no need to return true since there is no need to refer to the URL Metrics to apply the optimization. For example:

function add_decoding_async_to_img_tags( OD_Tag_Visitor_Context $context ): bool {
	if ( $context->processor->get_tag() !== 'IMG' ) {
		return false;
	}
	$context->processor->set_attribute( 'decoding', 'async' );
	return false;
}

So this PR retains the ability to return a boolean, but it also introduces a new track_tag method exposed on the supplied OD_Tag_Visitor_Context object which is equivalent to return true. Having a method makes the decision to opt-in to tracking much more explicit. This simplifies the above to:

function add_decoding_async_to_img_tags( OD_Tag_Visitor_Context $context ): void {
	if ( $context->processor->get_tag() !== 'IMG' ) {
		return;
	}
	$context->processor->set_attribute( 'decoding', 'async' );
}

And these are equivalent tag visitors:

function optimize_img( OD_Tag_Visitor_Context $context ): bool {
	if ( $context->processor->get_tag() !== 'IMG' ) {
		return false;
	}
	
	// Do optimizations based on $context->url_metric_group_collection...
	return true;
}

function optimize_img( OD_Tag_Visitor_Context $context ) {
	if ( $context->processor->get_tag() !== 'IMG' ) {
		return;
	}

	$context->track_tag();

	// Do optimizations based on $context->url_metric_group_collection...
}

For a bigger example of how this improves the DX, see this PR for the Intrinsic Dimensions PR: https://github.com/westonruter/od-intrinsic-dimensions/pull/1/files (cf. #10)