Expose Issues Completed under GraphQL flow metrics

What does this MR do and why?

Expose Issues Completed under GraphQL flow metrics. Counts the number of issues closed within the current time frame within the relevant scope (group or project). When they are opened is not considered. See !119423 (comment 1376030632) for details.

Changelog: added
EE: true

How to set up and validate locally

GraphQL query:

query {
  group(fullPath: "gitlab-org") {
    flowMetrics {
      issuesCompletedCount(from: "2023-01-01", to: "2023-05-03") {
        identifier
        value
        title
        links {
          label
          name
          url
          docsLink
        }
        unit
      }
    }
  }
}

You can also specify some filters, such as projectIds, milestones, assignees...

      issuesCompletedCount(from: "2023-01-01", to: "2023-05-03", projectIds: [1, 2], assigneeUsernames: ["cablett"], authorUsername: ["cablett"], milestoneTitle: "16.0", labelNames: "backend") {

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 #409330 (closed)