Add tracking for issues_mrs_empty_state experiment

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:

sequence	activity	category	action	label
	(Control/Candidate) User renders MR empty state page - question are we able to tell from where? ex. Nav, Learn GitLab, etc.	projects:merge_requests:index	render_project_mrs_empty_list_page	project_mrs_empty_list
	(Candidate) User clicks play on Merge Request video	-	-	-
	(Control/Candidate) User clicks New Merge Request or Create a new merge request button	projects:merge_requests:index	click_new_mr_project_mrs_empty_list_page	new_mr_project_mrs_empty_list
	(Candidate) User clicks Learn more about merge requests button	projects:merge_requests:index	click_learn_more_project_mrs_empty_list_page	learn_more_project_mrs_empty_list

For the Issues empty state:

sequence	activity	category	action	label
	(Control/Candidate) User renders Issues empty state page - question are we able to tell from where? ex. Nav, Learn GitLab, etc.	projects:issues:index	render_project_issues_empty_list_page	project_issues_empty_list
	(Candidate) User clicks play on Issue video	-	-	-
	(Control/Candidate) User clicks on Learn more about issues	projects:issues:index	click_learn_more_project_issues_empty_list_page	learn_more_project_issues_empty_list
	(Control/Candidate) User clicks New Issue or Create new issue button	projects:issues:index	click_new_issue_project_issues_empty_list_page	new_issue_project_issues_empty_list
	(Control/Candidate) User clicks on Email a new issue to this project or Email a new issue	projects:issues:index	click_email_issue_project_issues_empty_list_page	email_issue_project_issues_empty_list
	(Control/Candidate) User clicks on Import	projects:issues:index	click_import_csv_project_issues_empty_list_page / click_import_jira_project_issues_empty_list_page	CSV or JIRA
	(Control/Candidate) User clicks on JIRA integration	projects:issues:index	click_jira_int_project_issues_empty_list_page	jira_int_project_issues_empty_list

Related to #436480 (closed)