output: forget the metrics File inside the BufWriter (iopsystems#256)

The output file has a reference held by both the BufWriter
as well as the NamedTempFile, which results in both of them
attempting to close it and a fatal runtime error: "IO Safety
violation: owned file descriptor already closed".

Forget the file inside the BufWriter, so it is only closed
when the NamedTempFile is finished with it.

src/output/mod.rs

@@ -360,11 +360,18 @@ pub async fn metrics(config: Config) {
                 Snapshot::to_msgpack(&snapshot).expect("failed to serialize")
             }
         };
-        let _ = writer.write_all(&buf).await;
+        if let Err(e) = writer.write_all(&buf).await {
+            eprintln!("error writing metrics output: {}", e);
+            std::process::exit(1);
+        }
+    }
+
+    if let Err(e) = writer.flush().await {
+        eprintln!("error writing metrics output: {}", e);
+        std::process::exit(1);
     }
 
-    let _ = writer.flush().await;
-    drop(writer);
+    std::mem::forget(writer.into_inner());
 
     // Post-process metrics into a parquet file
     if metrics_config.format() == MetricsFormat::Parquet {