Add integration test for setting an alert

Summary

Following on our initial integration test for the Monitor stage features, we want to add more integration tests to safeguard our growing team from introducing regressions and to gain confidence that the core parts of our features continue to work even as the codebase is rapidly changing.

This integration test should cover:

1. User can set an alert from a chart on the metrics dashboard
2. Validating that it remains set in the UI, even on page refresh.
3. (optional) When the alert fires, it is reflected in the chart. This one may be too time-consuming or too complex to be worth it at this stage. We can determine this once the work is underway.