Duplicated and deprecated WPT metrics

[max-ostapenko]

WPT has a lot of duplicated/deprecated metrics:

• https://docs.webpagetest.org/metrics/page-metrics/
• https://docs.webpagetest.org/metrics/request-metrics/

We could review and clean these up to optimize the stored objects.

[max-ostapenko]

@pmeenan in case of duplicates do you have an idea which should we keep?
E.g.:

• loadTime - (also fullyLoaded) Time to fully load the page
• loadEventStart - (also chromeUserTiming.loadEventStart) Start time of the last main-frame load event (before the event handlers run)
• browser_version - (also browserVersion) Version string for the browser (i.e. 85.0.4183.102)

[max-ostapenko]

I listed some ideas preparing the docs update: HTTPArchive/har.fyi#15.