[Telemetry] Send an event when users view the new tab screen

[tdsmith]

Description & Product Manager / Data Scientist User Story

In order to understand whether we can improve the effectiveness of each messaging surface we’re testing in the set-default experiment (#18796), we should understand how many people see our messages at all. This relates to the branch described in #18375.

As a data scientist, I want to know how often users view the new-tab UI. I would like to observe a Glean event each time users view the new-tab UI.

There is a new tab tapped event. If there's no other way that users experience new tab, this new telemetry isn't a high priority.

What questions will you answer with this data?

How many users will see, saw, or could have seen the message we will place on new tab?

Acceptance Criteria

• ENG files a DS JIRA request outlining their methodology. Let's discuss in this ticket if you have any questions.
• DS sign off on instrumentation methodology addressing product questions.
• Event pings can be queried via re:dash
• Event pings can be queried via amplitude
• We are sending telemetry events for the actions listed in the requirements
• We have documented the telemetry
• We have asked a data steward to review the telemetry

┆Issue is synchronized with this Jira Task

[lobontiumira]

Verified as fixed on the 4/19 Nightly build with Google Pixel (Android 10).
Ping: de13230d-0cbb-4393-b51d-8fa11679e435

{
          "category": "home_screen",
          "name": "home_screen_displayed",
          "timestamp": 3333
        }

Glean dashboard
logcat.txt

[eliserichards]

PR: #19025