Add new metric for Quiet Mode status

[sstafford]

What feature do you want to see added?

I would like to monitor the QuietDown status of the Jenkins server. This will allow me to alert on cases where Jenkins is unintentionally left in quiet mode after automated maintenance is completed.

The Jenkins.isQuietingDown method can be used to determine the status:

• https://javadoc.jenkins.io/jenkins/model/Jenkins.html#isQuietingDown()
• https://github.com/jenkinsci/jenkins/blob/master/core/src/main/java/jenkins/model/Jenkins.java#L2982

Upstream changes

No response

Are you interested in contributing this feature?

Yes, I will submit a pull request.

[sstafford]

I have submitted a pull request to address this issue. There is a unit test for the code, and I also built the hpi file and deployed it to a Jenkins instance to confirm that the metric was available in the /prometheus endpoint and that the value changed when I enabled and disabled quiet mode in the Jenkins UI.

[Waschndolos]

merged with https://github.com/jenkinsci/prometheus-plugin/pull/687