The `secret` property of `configurationSchema` used a random `default`. That
expression is re-evaluated every time the module is loaded, and xo-server never
persists the values ajv fills in from schema defaults, so each restart handed
the metrics endpoint a brand new bearer token and Prometheus started getting
401s.
Drop the default and generate the secret in `load()` instead, saving it through
`xo.configurePlugin()` so it survives a restart.
Introduced by #9323
See https://xcp-ng.org/forum/topic/12415
fe29bd0d42