[resultui] track opt-in event
[resultui] track opt-in event

R=bryner, nodir, nqmtuan

Change-Id: I812ca76ca23cff76a5ded6966be6c452d70dc237
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2818366
Auto-Submit: Weiwei Lin <968518a24f41fef00b747498600e52b792d06ae0@google.com>
Reviewed-by: Brian Ryner <388ea68edb4d8f934960c128120aee09a8f854bf@google.com>
Commit-Queue: Weiwei Lin <968518a24f41fef00b747498600e52b792d06ae0@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go