Load dashboards from project's git repository

What does this MR do?

Closes #59974 (closed)

Checks dashboard param from querystring. If it exists, pass that to the metrics_dashboard endpoint.

Example URL for custom: http://localhost:3001/root/metrics/environments/36/metrics?dashboard=.gitlab/dashboards/super.yml

Depends on .gitlab/dashboards/super.yml file existing in repo. I copied contents from spec/fixtures/lib/gitlab/metrics/dashboard/sample_dashboard.yml

Feature flags to enable:

echo 'Feature.enable(:environment_metrics_use_prometheus_endpoint)' | bundle exec rails c
echo 'Feature.enable(:environment_metrics_show_multiple_dashboards)' | bundle exec rails c

Sreenshots