Merge branch 'tr-prometheus-metrics-dom-content-loaded' into 'master'

Move metrics initialization out of DOMContentLoaded

See merge request gitlab-org/gitlab!52293

app/assets/javascripts/pages/projects/prometheus/metrics/edit/index.js

-import customMetrics from '~/custom_metrics';
+import CustomMetrics from '~/custom_metrics';
 
-document.addEventListener('DOMContentLoaded', customMetrics);
+CustomMetrics();

app/assets/javascripts/pages/projects/prometheus/metrics/new/index.js

-import customMetrics from '~/custom_metrics';
+import CustomMetrics from '~/custom_metrics';
 
-document.addEventListener('DOMContentLoaded', customMetrics);
+CustomMetrics();