VSD - [BE] Adding "Total Flow items" metric (open + completed)

Overview

Following this beta feedback.

Problem to solve

Before we can calculate the "Flow throughput" metrics (completed/total), we need to count the Total Flow items.

Proposal

Measure the "Total Flow items" metric using virtual VSA stages (following this comment).

1. The collected data for lead time (issue created - issue closed) metric can be used to calculate the count. Instead of requesting the .median we can just call .count.
2. For the count total mrs completed, we could implement another virtual stage that tracks mr creation - mr merged time.

How to calculate "Total Flow items" metric

1. count total open issues
2. count total issues completed
3. count total open mrs
4. count total mrs completed
5. Sum 1-4