When waiting for metrics, GitLab doesn't tell us it fails
Summary
Related to #32777 (closed), when attempting to view metrics, we'll sometimes fail to talk to a prometheus server for X reason. The end user navigating to the metrics board won't realize something is wrong until they open up Dev Tools. This is a poor user experience.
Steps to reproduce
Configure a prometheus integration with a server that does not work. Then navigate to the metrics dashboard on a project.
What is the current bug behavior?
Nothing.
What is the expected correct behavior?
An error banner with reasons that are actionable to the user to mitigate in the future or helpful information to provide to support.
Relevant logs and/or screenshots
WIP design
The proposal is to introduce a warning message, letting users know that we've failed to connect to the Prometheus server. In addition, when this scenario occurs, we should update the text in the empty state from "waiting for performance data" to something that more accurately describes the error that's happening:
Output of checks
GitLab.com