Log Sourcegraph extensions events in BigQuery

Created by: ebrodymoore

Background: there's a huge effort going on right now build extensions as well as the UX around extension discoverability.

We have access to user settings for Cloud users but extensions are not really useful for these users so we've deprioritized this data. Instead, we've learned that almost all extensions usage across all instances (including Cloud) could be tracked. This would be huge to have this data to better inform engagement and extensions strategy. To do this, we need the Cloud team's help 😃

Context from @dadlerj:

The majority of self-hosted customer instances actually pull extensions from Sourcegraph.com/extensions (the clients pull this, so each user’s browser downloads each extension on every pageview), we SHOULD actually already have really good usage/download data… We should check on network access, and perhaps have some sort of backend logging on this.

How we log this—as a “backend event” through our event_logs table and into BigQuery, or as some server log that goes into Stackdriver/LogDNA—is up to the cloud team (and perhaps @felixfbecker / @ebrodymoore ). The volume of these pulls must be insanely gigantic though, so we should be ready for this to absolutely explode the size of our sourcegraph.com event_logs DB if we use that method… Perhaps we create some custom pipeline to log these in BigQuery without storing them in Postgres as well