add generic maybe_insert function

lib/vector-core/src/config/log_schema.rs

@@ -7,10 +7,6 @@ use vrl::path::parse_target_path;
 static LOG_SCHEMA: OnceCell<LogSchema> = OnceCell::new();
 static LOG_SCHEMA_DEFAULT: Lazy<LogSchema> = Lazy::new(LogSchema::default);
 
-const MESSAGE: &str = "message";
-const TIMESTAMP: &str = "timestamp";
-const HOST: &str = "host";
-const SOURCE_TYPE: &str = "source_type";
 const METADATA: &str = "metadata";
 
 /// Loads Log Schema from configurations and sets global schema. Once this is

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

@@ -7,6 +7,7 @@ use vector_common::{
     internal_event::TaggedEventsSent, json_size::JsonSize, request_metadata::GetEventCountTags,
     EventDataEq,
 };
+use vrl::path::{PathPrefix, ValuePath};
 
 use super::{
     BatchNotifier, EstimatedJsonEncodedSizeOf, EventFinalizer, EventFinalizers, EventMetadata,
@@ -84,13 +85,30 @@ impl TraceEvent {
         self.0.contains(key.as_ref())
     }
 
+    // TODO This should eventually use TargetPath for the `key` parameter.
+    // https://github.com/vectordotdev/vector/issues/7070
     pub fn insert(
         &mut self,
         key: impl AsRef<str>,
         value: impl Into<Value> + Debug,
     ) -> Option<Value> {
         self.0.insert(key.as_ref(), value.into())
     }
+
+    // TODO Audit code and use this if possible.
+    // https://github.com/vectordotdev/vector/issues/7070
+    #[allow(clippy::needless_pass_by_value)] // TargetPath is always a reference
+    pub fn maybe_insert<'a, F: FnOnce() -> Value>(
+        &mut self,
+        prefix: PathPrefix,
+        path: Option<impl ValuePath<'a>>,
+        value_callback: F,
+    ) -> Option<Value> {
+        if let Some(path) = path {
+            return self.0.insert((prefix, path), value_callback());
+        }
+        None
+    }
 }
 
 impl From<LogEvent> for TraceEvent {

src/transforms/remap.rs

@@ -486,9 +486,9 @@ where
                 }
             }
             Event::Trace(ref mut trace) => {
-                if let Some(metadata_key) = log_schema().metadata_key() {
-                    trace.insert(metadata_key.to_string(), self.dropped_data(reason, error));
-                }
+                trace.maybe_insert(PathPrefix::Event, log_schema().metadata_key(), || {
+                    self.dropped_data(reason, error).into()
+                });
             }
         }
     }