Add project count and coverage count to tooltip

What does this MR do and why?

For #263723 (closed)

On the Average test coverage last 30 days graph on the group repositories analytics page, this MR adds the number of projects (projectCount) and the number of coverage jobs (coverageCount) used to calculate the average coverage percentage (averageCoverage, the value that we're graphing over time) to the tooltip displayed when you hover over the graph.

Why? To make it easier to determine the cause of drops and spikes in the graph by comparing how many projects/jobs contributed to the average on certain days

Screenshots or screen recordings

before	after
Screen_Recording_2021-11-01_at_14.22.35	Screen_Recording_2021-11-01_at_14.12.36

How to set up and validate locally

1. have a GitLab Premium or higher license
2. git checkout 263723-add-stats-to-group-coverage-tooltip
3. find or create a group that contains projects that generate test coverage results

You can add this to a project's .gitlab-ci.yml to get a pipeline with any coverage value you'd like:

stages:
  - test

test:
  stage: test
  script:
    - echo COVERAGE 80%
  coverage: '/COVERAGE ([0-9]+)%/'

4. navigate to the group, then in the sidebar click Analytics -> Repository
5. hover over the Average test coverage last 30 days graph and the tooltip should show the project count and job count in addition to the average coverage value

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.