Productivity Analytics - Add stories to gitlab storybook

What does this MR do and why?

This MR adds storybook stories and different states for various vue components used on the productivity analytics page:

• metric_chart.vue
• metric_column.vue
• mr_table.vue
• mr_table_row.vue

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

How to set up and validate locally

# install storybook dependencies
$ yarn storybook:install

# start storybook
$ yarn storybook:start

Related to #428286 (closed)