Add comments re dropping and then readding fields

Signed-off-by: Stephen Wakely <fungus.humungus@gmail.com>

lib/vector-core/src/event/log_event.rs

@@ -292,6 +292,10 @@ impl LogEvent {
         }
     }
 
+    /// Retrieves the value of a field based on it's meaning.
+    /// This will first check if the value has previously been dropped. It is worth being
+    /// aware that if the field has been dropped and then some how readded, we still fetch
+    /// the dropped value here.
     pub fn get_by_meaning(&self, meaning: impl AsRef<str>) -> Option<&Value> {
         if let Some(dropped) = self.metadata().dropped_field(&meaning) {
             Some(dropped)

lib/vector-core/src/event/metadata.rs

@@ -133,6 +133,9 @@ impl EventMetadata {
     }
 
     /// Adds the value to the dropped fields list.
+    /// There is currently no way to remove a field from this list, so if a field is dropped
+    /// and then the field is re-added with a new value - the dropped value will still be
+    /// retrieved.
     pub fn add_dropped_field(&mut self, meaning: String, value: Value) {
         self.dropped_fields.insert(meaning, value);
     }