Add metadata command (#569)

* Add metadata command

* Add string interpolation to testing

crates/nu-protocol/src/pipeline_data.rs

@@ -37,12 +37,12 @@ pub enum PipelineData {
     Stream(ValueStream, Option<PipelineMetadata>),
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub struct PipelineMetadata {
     pub data_source: DataSource,
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub enum DataSource {
     Ls,
 }
@@ -52,6 +52,13 @@ impl PipelineData {
         PipelineData::Value(Value::Nothing { span }, None)
     }
 
+    pub fn metadata(&self) -> Option<PipelineMetadata> {
+        match self {
+            PipelineData::Stream(_, x) => x.clone(),
+            PipelineData::Value(_, x) => x.clone(),
+        }
+    }
+
     pub fn into_value(self, span: Span) -> Value {
         match self {
             PipelineData::Value(Value::Nothing { .. }, ..) => Value::nothing(span),