The essentials, and metrics plugins are core and critical to Essentials behaviour. There should be a smoke test with a clear message to understand quickly they are missing (and nothing much will work).

For the record:

• essentials plugin is absolutely critical to error logging. It takes Jenkins logs into the expected JSON logs file.
• metrics plugin is used for health-checking. So though for now, this only impacts tests, it is a critical component for when we implement the auto-upgrade.