[APM] Relax time-shifting so it doesn't affect async transactions

[sorenlouv]

Background: #24072

This is how we currently do time shifting:

kibana/x-pack/plugins/apm/public/components/app/TransactionDetails/Transaction/WaterfallContainer/Waterfall/waterfall_helpers/waterfall_helpers.ts

Lines 123 to 159 in af3594f

	export function getClockSkew(
	item: IWaterfallItem,
	parentItem?: IWaterfallItem
	) {
	if (!parentItem) {
	return 0;
	}
	switch (item.docType) {
	// don't calculate skew for spans. Just use parent's skew
	case 'span':
	return parentItem.skew;
	// transaction is the inital entry in a service. Calculate skew for this, and it will be propogated to all child spans
	case 'transaction': {
	const parentStart = parentItem.timestamp + parentItem.skew;
	const parentEnd = parentStart + parentItem.duration;
	// determine if child starts before the parent
	const offsetStart = parentStart - item.timestamp;
	// determine if child starts after the parent has ended
	const offsetEnd = item.timestamp - parentEnd;
	// child transaction starts before parent OR
	// child transaction starts after parent has ended
	if (offsetStart > 0 || offsetEnd > 0) {
	const latency = Math.max(parentItem.duration - item.duration, 0) / 2;
	return offsetStart + latency;
	// child transaction starts withing parent duration and no adjustment is needed
	} else {
	return 0;
	}
	}
	}
	}

Time shifting in its current form may be causing more harm than good, especially for users using async message queues.

Solution could be to only time shift when child start before parent. Not if the child starts after parent has ended.

[elasticmachine]

Pinging @elastic/apm-ui