The `datanodeStorage` field belongs directly under `monitoring.standalone`
(GreptimeDBStandaloneSpec), not under `monitoring.standalone.base`
(PodTemplateSpec). Nesting it under `base` renders an invalid
GreptimeDBCluster resource and the CRD schema rejects it during
server-side apply:
.spec.monitoring.standalone.base.datanodeStorage: field not declared in schema