Add File System Observer

[tomayac]

Add File System Observer custom metric check

[max-ostapenko]

Reason for test error - the WPT test request needs to be forced to run as POST.
Adjusted in #136

[github-actions]

Custom metrics for https://almanac.httparchive.org/en/2022/

WPT test run results: http://webpagetest.httparchive.org/results.php?test=240722_Q1_4
Changed custom metrics values:

{
  "_fugu-apis": {
    "Device Memory": [
      "https://almanac.httparchive.org/static/js/send-web-vitals.js?v=b7224f484fe762e075d4838286ddb066"
    ]
  }
}