What would be the correct way, if at all, to add a custom metric that measures usage of EXIF attributes in images on the web (if such metric doesn’t already exist?)
The use case: https://discourse.wicg.io/t/proposal-exif-image-resolution-auto-and-from-image/4326
EXIF is now supported in browser specs in the image-orientation CSS attribute, and we’re looking for a way to assess the effect of supporting more EXIF attributes on existing websites.
To answer the broader question, custom metrics are implemented using snippets of JS that are added to the custom_metrics directory of the test server on GitHub. The meat of your question is about how to write a JS snippet to analyze EXIF attributes, which I’m not sure about. Maybe @patmeenan or @colin know?
You can’t do it from JS. It would be an agent modification to wptagent to scan for whatever we’re specifically looking for. It already scans the JPEG blocks to count the progressive scans but it doesn’t do anything with EXIF yet.