The new header will be recognized starting with 1.25wmf10, see
With Gerrit change 174200, an HTTP header "Api-User-Agent" header will be recognized for logging from the API. This should benefit clients using XMLHttpRequest where the normal User-Agent header is locked down by the browser. The logged agent will be the concatenation of Api-User-Agent and the browser's User-Agent header.
This should come in handy once the ApiFeatureUsage extension is reviewed and deployed.