Glean client_info.app_channel in Firefox Desktop reports Update Channel starting Firefox 90

[Chris H-C]

Hi!

  With https://bugzilla.mozilla.org/show_bug.cgi?id=1710664 the value for client_info.app_channel reported by Firefox Desktop will now be the default value of the app.update.channel pref (the Update Channel) instead of the build configuration's idea of the app's channel (the Build Channel).

  The impetus behind this is to ensure our population of Firefox Beta users don't look like Release users when we ship Release Candidate builds to them. You can see this at [1] where "beta" gets donated to "release" for about 10 days roughly every four weeks (matching the Release Calendar). Starting with Firefox 90, our beta population should remain consistent even across those ten days.

  Beta 89's RC builds (beginning soon) are expected to show the same sort of donation dip. But they should be the last.

  So far on nightly[1] the only effect has been to allow some odder values like `night5555ly` into the data (as always the normalized_channel column takes care of the weird ones). If there is no default value for the app.update.channel pref, the value of client_info.app_channel will be `unknown` (which normalized_channel will normalize to "Other").

  I'll be keeping an eye on this as it rides the trains. If you run into any problems or have any questions, please do let me know in reply to this or by finding me on Matrix[2].

:chutten

[1]: https://sql.telemetry.mozilla.org/queries/79269/source#196919

[2]: https://chat.mozilla.org/#/room/#glean:mozilla.org