Record custom element extension version numbers in AMP4ADS response.

Currently AMP4ADS documents have a JSON field that lists the set of custom element extensions in the document.

For example, if a document contains:

<script async custom-element='amp-analytics' src='https://cdn.ampproject.org/v0/amp-analytics-0.1.js'></script>
<script async custom-element='amp-list' src='https://cdn.ampproject.org/v0/amp-list-0.1.js'></script>

The AMP4ADS json will include:

"customElementExtensions" : [ "amp-analytics", "amp-list" ]

In the future, it will become important to also include the version numbers used in these extensions in some form. Currently they are (almost) all version 0.1 only.

My first suggestion is to actually capture the entire attribute values, rather than parse out the version.

"customElementExtensions" : [
   {
      "custom-element": "amp-analytics",
      "src": "https://cdn.ampproject.org/v0/amp-analytics-0.1.js"
   },
   {
      "custom-element": "amp-list",
      "src": "https://cdn.ampproject.org/v0/amp-list-0.1.js"
   },
]

This approach can also handle version labels such as amp-list-latest.js.

[Gregable]

Ideally you are already sending down:

"customElementExtensions" : [ "amp-analytics", "amp-list" ]

You should also now send down:

"customElements" : [
   {
      "custom-element": "amp-analytics",
      "src": "https://cdn.ampproject.org/v0/amp-analytics-0.1.js"
   },
   {
      "custom-element": "amp-list",
      "src": "https://cdn.ampproject.org/v0/amp-list-0.1.js"
   },
]

And at some point in the future, you can remove the former if you’d like.

[Gregable]

My understanding is that the client side implementation was here: https://github.com/ampproject/amphtml/pull/8381

And this has been implemented for a long time, but was never closed. Feel free to reopen if I am missing something.