- record number of saved tab groups
- record min/max/median/average number of tabs in saved tab groups
This patch adds a new observable topic `sessionstore-saved-tab-groups-changed` to SessionStore. This is emitted when a saved tab group is added or removed from the session. BrowserUsageTelemetry subscribes to that topic and updates saved tab group-related metrics.
BrowserUsageTelemetry is also setting those saved tab group-related metrics on startup. BrowserGlue initializes BrowserUsageTelemetry after the session has been restored/set up, so saved tab group information will be available at that time. This is also important to make sure that we keep this metric correct even if a user doesn't touch their saved tab groups during the course of a session.
Differential Revision: https://phabricator.services.mozilla.com/D245138
8551cfa4aa