Added iframes custom_metric

[kevinfarrugia]

This is a custom metric that retrieves the data related to <iframe> elements present on the page, including their attributes.

It will be used in the 2021 Resource Hints chapter to retrieve lazy loaded iframes.

Sample WPT https://webpagetest.org/custom_metrics.php?test=210526_BiDcXM_dbe65778982f32e4a5df707fa05d5d74&run=1&cached=0

Data

{
  "iframes": {
    "total": 2,
    "nodes": [
      {
        "tagName": "iframe",
        "loading": "lazy",
        "width": "560",
        "height": "315",
        "src": "https://www.youtube.com/embed/XBfYPp6KF2g",
        "allow": "accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture",
        "allowfullscreen": ""
      },
      {
        "tagName": "iframe",
        "loading": "lazy",
        "width": "560",
        "height": "315",
        "src": "https://www.youtube.com/embed/rrVDATvUitA",
        "allow": "accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture",
        "allowfullscreen": ""
      }
    ],
    "attribute_usage_count": {
      "loading": 2,
      "width": 2,
      "height": 2,
      "src": 2,
      "frameborder": 2,
      "allow": 2,
      "allowfullscreen": 2
    }
  },
  "loading_values": ["lazy", "lazy"]
}

[kevinfarrugia]

Closed. Duplicate of #201