Adds KPI For Customer Engagement On Gitlab Issues

Why

• It was requested per Product#541 (closed)
• In creating an Issue for the Data team, the template suggested that all KPIs and metrics should live in the handbook.

What

• Adds a WIP Customer Engagement On GitLab Issues metric to Product in the KPI index.

Related Issues

• KPI Implementation Issue (https://gitlab.com/gitlab-data/analytics/issues/2863)