For logging to work:
1. $wgWMEStatsdBaseUri needs to point to a valid statsv endpoint,
e.g. 'https://en.wikipedia.org/beacon/statsv'.
2. $wgPopupsStatsvSamplingRate needs to be set. Note that the codebase
already contains the EventLogging functionality, which is configured
separately. Separately configuring different logging mechanisms
allows us to avoid sampling mistakes that may arise while choosing
one or the other. For example, let's say we want to use EventLogging for
10% of users and statsv for 5%. We'd sample all users into two
buckets: 50/50. And then we'd have to set the sampling rates as
20% and 10% respectively, only because of the bucketing above. To avoid
this kind of complications, separate sampling rates are used for each
logging mechanism. This, of course, may result in situations where a
session is logged via both EventLogging and statsv.
3. The WikimediaEvents extension needs to be installed. The extension
adds the `ext.wikimediaEvents` module to the output page. The
logging functionality is delegated to this module.
Notable changes:
* The FETCH_START and FETCH_END actions are converted to a timed action.
* The experiments stub used in tests has been extracted to the stubs
file.
Logged data is visualized at
https://grafana.wikimedia.org/dashboard/db/reading-web-page-previews
Bug: T157111
Change-Id: If3f1a06f1f623e8e625b6c30a48b7f5aa9de24db
