Recording Telemetry scalars from add-ons

The Go Faster initiative is important as it enables us to ship code faster, using special add-ons, without being strictly tied to the Firefox train schedule. As Georg Fritzsche pointed out in his article, we have two options for instrumenting these add-ons: having probe definitions ride the trains (waiting a few weeks!) or implementing and sending a new custom ping (doing some pipeline work!).

Both solutions are not very appealing when shipping code faster. But hey.. we have plan!

Our current work is focused on extending Telemetry to fill this gap. The first step consisted in enabling add-ons event recording in Firefox 56 (bug) and we recently enabled add-on scalar recording as well (bug)!

…