Remove loadHTMLFixture usage in `spec/frontend/prometheus_metrics/`
This MR aims to solve #405325 (closed)
Replace references to loadHTMLFixture
in the following places:
spec/frontend/prometheus_metrics/custom_metrics_spec.js:20: loadHTMLFixture(FIXTURE);
spec/frontend/prometheus_metrics/prometheus_metrics_spec.js:14: loadHTMLFixture(FIXTURE);