vars as data attribute does not work on amp-story-page element

### Description

I'm trying to send analytics to GA. I've created a `var as data attribute` like is and place it on `amp-story-page` like it's written here: https://github.com/ampproject/amphtml/blob/main/extensions/amp-analytics/analytics-vars.md#variables-as-data-attribute.

but it doesn't seem to work.
All the other parameters are working.


### Reproduction Steps

1. create `amp-story-page`
2. add a data attribute
3. try to send it to google analytics

### Relevant Logs

_No response_

### Browser(s) Affected

Chrome

### OS(s) Affected

_No response_

### Device(s) Affected

_No response_

### AMP Version Affected

_No response_

Start with extensions/amp-analytics/analytics-vars.md and the amp-story-page reproduction described in the issue. Trace how amp-analytics reads data attributes on story pages, then reproduce the Google Analytics request in Chrome. Done means the documented attribute is recognized on amp-story-page and its value reaches the analytics request.

Written by the indexing model from the issue text.