Feature Description

The Server Timing API in the plugin currently exposes metrics wp-before-template and wp-template, which when added make up for the total response time. While it's easy to calculate the sum of the two, when running many performance tests in an automated manner, this is not realistically feasible.

We should avoid using the sum of the two medians as that is statistically incorrect. We need to instead have the total load time in every run and use the median of that.

A wp-total metric was already included in the WordPress core automated performance CI workflow (see https://core.trac.wordpress.org/browser/trunk/tests/performance/wp-content/mu-plugins/server-timing.php?rev=55459#L26), so we should implement that in the Performance Lab plugin too.