For #2267: Adds telemetry for reader mode

[sblatz]

Pull Request checklist

• Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry or does not need one
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

[sblatz]

Request for data collection review form

All questions are mandatory. You must receive review from a data steward peer on your responses to these questions before shipping new data collection.

1. What questions will you answer with this data?

• How often users switch to reader mode?
• How often users switch to reader mode, on sites where reader mode is available?
• How often users tap on the Appearance icon to change the font, etc?

2. Why does Mozilla need to answer these questions? Are there benefits for users? Do we need this information to address product or business requirements?

• To measure how useful these features are

3. What alternative methods did you consider to answer these questions? Why were they not sufficient?

• N/A (These are baseline metrics)

4. Can current instrumentation answer these questions?

• Currently no, as these are some of the first metrics we're recording

5. List all proposed measurements and indicate the category of data collection for each measurement, using the Firefox data collection categories on the found on the Mozilla wiki.

• All data is Category 2.

6. How long will this data be collected?

Until 04/01/2020

7. What populations will you measure?

• All release, beta, and nightly users with telemetry enabled.

8. Please provide a general description of how you will analyze this data.

• Glean / Amplitude

9. Where do you intend to share the results of your analysis?

• Only on Glean, Amplitude and with mobile teams.

[boek]

Data Review Form (to be filled by Data Stewards)

Instructions: Data Stewards will review a request for data collection and endorse responses to each question.

1. Is there or will there be documentation that describes the schema for the ultimate data set in a public, complete, and accurate way?
   Yes, with metrics.yaml and metrics.md

2. Is there a control mechanism that allows the user to turn the data collection on and off?
   Yes, in the Fenix settings

3. If the request is for permanent data collection, is there someone who will monitor the data over time?
   Has expiry, Fenix team will monitor

4. Using the category system of data types on the Mozilla wiki, what collection type of data do the requested measurements fall under?
   All data is category 2.

5. Is the data collection request for default-on or default-off?
   Default on

6. Does the instrumentation include the addition of any new identifiers (whether anonymous or otherwise; e.g., username, random IDs, etc. See the appendix for more details)?
   No

7. Is the data collection covered by the existing Firefox privacy notice?
   yes

8. Does there need to be a check-in in the future to determine whether to renew the data?
   Has expiry. Fenix team will monitor

9. Does the data collection use a third-party collection tool?
   No