This adds probes and a limited set of tests for the following types of measurements:
1. A count for each time the All Tabs panel is opened, keyed on the entrypoint.
2. A count for each usage of the Tab Context Menu, keyed on the trigger node
as the entrypoint.
3. A count for each dragstart within the All Tabs panel.
Differential Revision: https://phabricator.services.mozilla.com/D168305
