== Background
In principle, the view-based metrics planned for AMC (see {T210660}) can be determined from the data that's collected in the [[https://wikitech.wikimedia.org/wiki/Analytics/Data_Lake/Traffic/Webrequest | `wmf.webrequest` Hive table ]] provided that we can distinguish between regular users and those users who've enabled AMC.
We already achieve the same for users who have enabled Wikipedia's mobile beta mode by adding a well-known key-value pair to [[ https://wikitech.wikimedia.org/wiki/X-Analytics | the X-Analytics header ]]: https://github.com/wikimedia/mediawiki-extensions-MobileFrontend/blob/3231dee54e6abc08080627f64157848542dd323d/includes/MobileContext.php#L1022-L1034
== Acceptance Criteria
[] Assess privacy implications
[] Add key-value pair to X-Analytics header when the user has enabled AMC
[] Document the key-value pair
== Notes
1. MobileFrontend's server-side manipulation of the X-Analytics header is already under test (see `MobileContext::addAnalyticsLogItem`).
# The value of the X-Analytics //response// header is logged and processed by WMF's analytics pipeline automatically, i.e. if a key-value pair is added to the header, it'll be reflected in the `wmf.webrequest` Hive table once it's been processed.
# `MobileContext::logMobileMode` exists and seems like the appropriate place to add this change.
# As @tbayer notes in T212961#4861533, it might be easiest to add a new value to the `mf-m` X-Analytics key, e.g. when the user has enabled both the mobile beta mode and AMC, we set the value to `mf-m=b,amc` (the other possible values being `=b`, `=amc`).