createHeaders.gs

function createHeaders() {
   //Freezes the first row
  sheet.setFrozenRows(1);
  // Set the values we want for headers
   var values = [["Timestamp", "Device","Performance Score","LAB FCP", "LAB LCP", "LAB CLS", "LAB Interactive", "LAB Total Blocking Time", "LAB Speed Index"]];
  // Set the range of cells
  var range = sheet.getRange(1, 1, 1, 9);
  //Call the setValues method on range and pass in our values
  range.setValues(values);
}