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);