VSD - Add MR throughput metrics

What does this MR do and why?

Updates the Value streams dashboard to include MR throughput data

• Adds a count of total MRs merged in the time period
• Updates docs
• Updates related tests

Note:

• For a project the link points to MR analytics with a 180 date range pre-filled
• For a group/sub-group the link points to productivity analytics, we don't have MR analytics at the group level

Before	After	After - tooltip

How to set up and validate locally

You will need a GitLab Ultimate license

1. Create a group
2. Seed cycle analytics data
3. Seed dora metrics data
4. Seed MRs in the merged state - see this note for some hints
5. Visit the Value streams dashboard

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #383665 (closed)