FYI see also this thread.

For whoever works on this, to avoid confusion MobileFrontend does provide a hook for instrumentation in the legacy parser but NOT parsoid version which is fired every time a section is expanded or collapsed that may be needed as part of the instrumentation you are working on. (mw.hook( 'mobileFrontend.section-toggled' ))

Discussed and finalized metrics with @jwang. Measurement plan and instrumentation spec will be tracked in T400185: Set up measurement plan and instrumentation spec for image browsing cc @SherryYang-WMF

Measurement plan and instrumentation spec will be tracked in T409163