What the application itself says about its health: the metrics its own MetricsController answers (GET api/monitoring/metrics.json) -- php and opcache, the packages and their versions, and whatever the application counts (users, documents, jobs waiting...) through its providers.
class MMO\admin\monitoring\probes\ApplicationProbe extends MMO\admin\monitoring\aProbe
The route answers a token only, sent as `X-Hub-Token`: a target's `metricsToken` in the hub's credentials file, or
the one of `*` for every target. A target without one is not asked.
Metrics: the application's, prefixed `app.` (`app.php.memoryPeakMb`, `app.users.active`...). Facts: the
application's, prefixed the same. Its checks (`{name, status, message}`) make the reading's status.