[[mw:Extension:ImageMetrics]] will be used to collect additional data about image performance and (eventually - not implemented yet) about how users interact with media. It does not alter the behavior of the wiki, so there should be no need for community consensus or design review.
Security summary: all data collection is done via EventLogging, and data will be used internally or published via limn.
Performance summary: the extension has a similar architecture to NavigationTiming, except it lazy-loads the main script. Currently it consists of a ~300 byte top script which sets up pre-domready event handling, a ¬300-byte bottom script which deals with request sampling, and a ~2K main script which is lazy-loaded when the request is sampled and uses EventLogging to log image load performance data. Currently the scripts are only loaded on file pages, but I intend to change that as more functionality is added.
The code has already been reviewed internally by the Multimedia team.
After deployment, $wgImageMetricsSamplingFactor should be set to a suitable value (1000, probably).
Related Mingle card: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/816