Enable 1:1000 attribution logging for MediaViewer

Enable logging (via EventLogging) of attribution metadata retrieval
success rates for the thumbnails MediaViewer displays to the reader,
with 1:1000 sampling. Total logging frequency is number of
MediaViewer image views (~20M/day currently) / 1000.

Change-Id: I356a714ee376493f9699f979677b23725cdcfba9
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/858