Folder.getBuildHealthReports looks like it is double-counting each child Folder. It does a recursive traversal of the folder, applying FolderHealthMetric.Reporter to each node (leaf or not). But a typical FolderHealthMetric like WorstChildHealthMetric also calls getHealthReport on the child Folder, which winds up calling Folder.getBuildHealthReports recursively! This seems like a mistake.

Possible fixes:

• Make WorstChildHealthMetric.ReporterImpl.observe (and any other similar impls) ignore a Folder it is given.
• Make Folder.getBuildHealthReports not ask a reporter to observe a Folder.
• Make Folder.getBuildHealthReports not do a recursive traversal, only asking about direct children.

My preference is for the third fix, as it leaves the reporter in control of the logic; if it needs a recursive call it can do one.

By the way, the awkward reflection code in getHealthReport suggests that we need an interface in Jenkins core, such as

interface ItemWithHealthReport extends Item {
    HealthReport getBuildHealth();
    List<HealthReport> getBuildHealthReports();
}