Add alerting for RadosGW Usage Exporter metrics

[MoteHue]

No description provided.

[gemini-code-assist]

Code Review

This pull request adds a new Prometheus alert, RadosGWUsageExporterNotServingMetrics, to monitor the RadosGW Usage Exporter, and includes a corresponding release note. My review identified a critical issue with the Prometheus rule definition that will result in an invalid file, preventing Prometheus from loading it. I have also noted a minor typo in the release note. Please see the detailed comments for suggestions on how to resolve these issues.

[MoteHue]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a new Prometheus alert, RadosGWUsageExporterNotServingMetrics, to monitor the RadosGW Usage Exporter, along with a corresponding release note. The new alert is a valuable addition for ensuring the reliability of RGW usage metrics collection. However, I've identified a critical issue with how the new alert rule is added to the configuration file, which will result in an invalid YAML file and cause Prometheus rule loading to fail. My review includes a detailed explanation of the issue and guidance on how to resolve it.

[priteau]

We should extend this to all our critical metrics.