Count distinct authors of merge requests metric

What does this MR do and why?

Contributes to #379739 (closed)

Problem

We don't have metric that presents number of distinct authors of merge request per month, but it's requested by Customer Success team.

Solution

Add metric to count distinct authors of merge requests per month.

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.