Extend performance JSON to support metrics where lower is better

We were able to ship the MVC of browser performance metrics in 10.3, which was great. However this is currently limited to metrics like the performance score, where higher numbers are better.

This limits us from utilizing other metrics that are collected in the performance tests, such as: page size, number of requests, and more.

For example, findings from https://about.gitlab.com:

• The page has 4 requests inside of head that can cause a SPOF. Load them asynchronously or move them outside of document head.
• The page has 65 request(s) that are missing a cache time. Configure a cache time so the browser doesn't need to download them every time. It will save 2.2 MB the next access.
• The page total transfer size is 3.7 MB

We could solve this by adding an additional parameter to indicate whether a higher or lower value is better.