Function error_class 

pub fn error_class(code: Code) -> &'static str

Maps a gRPC status code to the QueryAuditRecord.error_class bucket used both by the audit log and to gate the query_failed/ query_duration_with_error metrics. This is the derived mapping used for every DataFusionError-rooted failure; the query-deny-list check stamps a fourth value, "denied", directly via QueryAuditState::fail_with_class rather than through this function, since a denial’s ResourceExhausted code would otherwise land in the "resource" bucket alongside a genuine resource-budget failure.