See https://ithoughthecamewithyou.com/post/automate-google-pagespeed-insights-with-apps-script

PageSpeed.gs

var pageSpeedApiKey = '';
var pageSpeedMonitorUrl = '';

function monitor() {
  var desktop = callPageSpeed('desktop', pageSpeedMonitorUrl);
  var mobile = callPageSpeed('mobile', pageSpeedMonitorUrl);

  var desktopVitals = getVitals(desktop);
  var mobileVitals = getVitals(mobile);

  var spreadsheet = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = spreadsheet.getSheetByName('results');

  sheet.appendRow([
                   Utilities.formatDate(new Date(), 'GMT', 'yyyy-MM-dd'),
                   desktop.lighthouseResult.categories.performance.score * 100,
                   desktopVitals.lcp == -1 ? ' ' : desktopVitals.lcp,
                   desktopVitals.fid == -1 ? ' ' : desktopVitals.fid,
                   desktopVitals.cls == -1 ? ' ' : desktopVitals.cls,
                   mobile.lighthouseResult.categories.performance.score * 100,
                   mobileVitals.lcp == -1 ? ' ' : mobileVitals.lcp,
                   mobileVitals.fid == -1 ? ' ' : mobileVitals.fid,
                   mobileVitals.cls == -1 ? ' ' : mobileVitals.cls
                  ]);
    
    // much more available, i.e. desktop.pageStats.numberResources
}

function callPageSpeed(strategy, url) {
  var pageSpeedUrl = 'https://www.googleapis.com/pagespeedonline/v5/runPagespeed?url=' + url + '&key=' + pageSpeedApiKey + '&strategy=' + strategy;
  var response = UrlFetchApp.fetch(pageSpeedUrl);
  var json = response.getContentText();
  return JSON.parse(json);
}

function getVitals(result) {
  var lcp;
  var fid;
  var cls;

  try {
    lcp = result.originLoadingExperience.metrics.LARGEST_CONTENTFUL_PAINT_MS.percentile / 1000;
  } catch {
    lcp = -1;
  }

  try {
    fid = result.originLoadingExperience.metrics.FIRST_INPUT_DELAY_MS.percentile / 1000;
  } catch {
    fid = -1;
  }

  try {
    cls = result.originLoadingExperience.metrics.CUMULATIVE_LAYOUT_SHIFT_SCORE.percentile;
  } catch {
    cls = -1;
  }

  return { "cls":cls, "fid":fid, "lcp":lcp  };
}