Based on
https://github.com/jenkinsci/metrics-plugin/blob/master/src/main/java/jenkins/metrics/api/Metrics.java#L118
the checks are put in a thread pool size 4, with no queue, with the HealthChecker itself taking one spot. If more than 3 checks are put in the pool before one finishes a RejectedExecutionException is thrown, never logged anywhere, and nothing is returned from registry.runHealthChecks