[Meta] Improvements for Browser Performance Testing

We have shipped an initial MVC in %10.3 of browser performance testing, indicating the direction we plan to go. We have quite a few improvements we would like to make to this feature in the coming releases, to really improve the value and ease of use:

1. Support additional metrics per page, like time to first paint and page size (https://gitlab.com/gitlab-org/gitlab-ee/issues/4370)
2. Extend JSON format to support metrics where lower is better, like: time to first load, number of requests, etc. (https://gitlab.com/gitlab-org/gitlab-ee/issues/4367)
3. Enable support for including more than just the JSON report in the artifact zip (https://gitlab.com/gitlab-org/gitlab-ee/issues/2877)
4. Single click from metric in MR widget to HTML artifact with more information (https://gitlab.com/gitlab-org/gitlab-ee/issues/4366)
5. Hide metrics that have not changed (https://gitlab.com/gitlab-org/gitlab-ee/issues/4369)

We hope to tackle these in the short term to improve the functionality of not just performance metric analysis, but also more broadly all of our MR analytics.