VSD - [BE] add "Velocity" metric (Flow items completed)

Overview

Flow Velocity is measure of productivity - a metric that measures how productive a process actually is. In practice, this means tracking how many items or units of work are completed within a specific period of time.

This is backend for VSD - [FE] add "Closed issues" (FKA "Velocity")... (#386715 - closed)

Following this beta feedback.

Proposal

1. Adding new metric "Velocity" to VSD comparison panel.
2. Metric definition - Monthly number of flow items completed - Issues & MRs
3. Data query via the GraphQL.
4. Later on the frontend the metric need to be filtered by label.

How "Flow Velocity" calculated

In a given time period:

1. count total issues completed
2. count total mrs completed
3. sum 1+2 = total flow items completed = "Velocity"

Implementation suggestion: #396934 (comment 1318715780)