What does this MR do and why?

Add tracking for issues_mrs_empty_state experiment

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

!147661 (merged)

How to set up and validate locally

bin/rails c

Feature.enable(:issues_mrs_empty_state)

• Enable incoming_email: in gitlab.yml.
• Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK
• Sign in. Create new group and blank project under that group.
• Visit project MRs list.
• Mare sure frontend events from the table below are generated.
• Visit project issues list.
• Mare sure frontend events from the table below are generated.

For the MRs empty state:

For the Issues empty state:

Related to #436480 (closed)