Enable population of event metadata by a VRL source

At the moment, there seems to be no way to produce an event with
non-trivial metadata for a unit test, which makes it hard to test
configurations that have log_namespace = true.

Reference: vectordotdev#19639.

src/config/unit_test/mod.rs

@@ -32,7 +32,7 @@ use crate::{
         self, loading, ComponentKey, Config, ConfigBuilder, ConfigPath, SinkOuter, SourceOuter,
         TestDefinition, TestInput, TestInputValue, TestOutput,
     },
-    event::{Event, LogEvent, Value},
+    event::{Event, EventMetadata, LogEvent, Value},
     signal,
     topology::{builder::TopologyPieces, RunningTopology},
 };
@@ -581,7 +581,12 @@ fn build_input_event(input: &TestInput) -> Result<Event, String> {
                 result
                     .program
                     .resolve(&mut ctx)
-                    .map(|v| Event::Log(LogEvent::from(v.clone())))
+                    .map(|_| {
+                        Event::Log(LogEvent::from_parts(
+                            target.value.clone(),
+                            EventMetadata::default_with_value(target.metadata.clone()),
+                        ))
+                    })
                     .map_err(|e| e.to_string())
             } else {
                 Err("input type 'vrl' requires the field 'source'".to_string())

tests/behavior/transforms/vrl_test_input.toml

@@ -17,3 +17,32 @@
     [[tests.outputs.conditions]]
       type = "vrl"
       source = """.a.b == "c" && is_timestamp(.d)"""
+
+##------------------------------------------------------------------------------
+
+[transforms.remap_metadata]
+  inputs = []
+  type = "remap"
+  source = '''
+    .event = .
+    .metadata = %
+  '''
+
+[[tests]]
+  name = "remap_metadata"
+  [[tests.inputs]]
+    insert_at = "remap_metadata"
+    type = "vrl"
+    source = '''
+      . = "Event"
+      % = "Metadata"
+    '''
+
+  [[tests.outputs]]
+    extract_from = "remap_metadata"
+    [[tests.outputs.conditions]]
+      type = "vrl"
+      source = '''
+        assert_eq!(.event, "Event")
+        assert_eq!(.metadata, "Metadata")
+      '''