Document private label value storage

Author: xudong963

datafusion/physical-expr-common/src/metrics/mod.rs

@@ -565,6 +565,9 @@ impl Display for Label {
 pub struct LabelValue(LabelValueInner);
 
 /// Internal representation for label names and values.
+///
+/// Keep the storage variants private so `LabelValue` can remain a public input
+/// type for `Label::new` without exposing its representation as public API.
 #[derive(Clone)]
 enum LabelValueInner {
     Static(&'static str),