Empty Stats: Event tracking for Reader discover nudge

[nikola-milicevic]

Part of #17307

This PR adds tracking events for Reader discover nudge.

Test

1. Fire the events from the table below
2. Make sure they fire only once
3. Validate the events: PCYsg-ktp-p2

Events table

Event	Trigger
stats_reader_discover_nudge_shown	Fired every time user goes to My Site > Stats screen and Reader discover nudge is shown
stats_reader_discover_nudge_tapped	Tap Discover blogs to follow on Reader discover nudge card
stats_reader_discover_nudge_dismissed	Tap Dismiss on Reader discover nudge card
stats_reader_discover_nudge_completed	Fired when user taps Done on Follow topics screen, while having selected at least one topic to follow

Regression Notes

1. Potential unintended areas of impact
   n/a

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   n/a

3. What automated tests I added (or what prevented me from doing so)
   n/a

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[momo-ozawa]

Works as described! 🎉
I validated the events on Tracks 😄

2021-10-22 11:26:06:637 WordPress[50415:3352932] 🔵 Tracked: stats_reader_discover_nudge_shown <blog_id: 198007634, first_nudge: publicize, site_type: blog>
2021-10-22 11:26:10:945 WordPress[50415:3352932] 🔵 Tracked: stats_reader_discover_nudge_dismissed <blog_id: 198007634, first_nudge: publicize, site_type: blog>
2021-10-22 11:27:38:636 WordPress[50415:3352932] 🔵 Tracked: stats_reader_discover_nudge_tapped <blog_id: 189057154, first_nudge: publicize, site_type: blog>
2021-10-22 11:27:43:393 WordPress[50415:3352932] 🔵 Tracked: stats_reader_discover_nudge_completed <blog_id: 189057154, first_nudge: publicize, site_type: blog>