Add total flow items for groups VSD

What does this MR do and why?

Backend part to include total flow items metrics for groups value streams dashboard.

The metric is the sum of:

1. total open issues
2. total issues closed
3. total open mrs
4. count total mrs merged

More details at #396963

How to set up and validate locally

The metric is not present in the UI yet, but it is possible to verify it by opening a group VSD page and checking the browser API calls responses.

1. Make sure to have an ultimate license
2. Create a group with a project
3. Open some issues and MRs within the project
4. Close some of the opened issues and merge some of the MRs and run Analytics::CycleAnalytics::ReaggregationWorker.new.perform on rails console
5. Execute the following query at GraphiQL explorer:

 query groupTotalFlow{
  group(fullPath: "GROUP_PATH") {
    flowMetrics {
      totalFlow(from: "2023-03-1", to: "2023-06-1") { // make sure dates match issuables creation/closing
        value
        identifier
        title
        unit
      }
     }
   }
 }

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.