Instrument toolbar button to measure click performance

[lmorchard]

Splitting out from #1036:

• including separate logging name to compare badged vs unbadged performance

[lmorchard]

Two potential spots for measurement:

1. Clicks on the toolbar button itself
2. Clicks on the listed experiments in the panel revealed by the button

Do we need to find ways to measure both? They will probably need to hook into different metrics systems - Telemetry for 1 and Google Analytics for 2

[lmorchard]

Word today is we want to measure clickthroughs on experiments, not button clicks.

So, we need Google Analytics params for each experiment varied on badged & non-badged. We can add those to experiment links in the panel.

[lmorchard]

Toolbar panel links are GA parameters are built here.

Plan is to change utm_content to badged and not badged as appropriate to what state the toolbar button had when it was clicked.

Going to say this unblocks work on the issue.

This will need inclusion in GA spec docs for #696