fmt/v2: extract metadata gathering into function

Extract the piece of code that gathers the metadata from the
result struct into its own (nested) method. It is easier to
read but also prepares for a future change where we read the
metadata from the store instead of the result dict.

osbuild/formats/v2.py

@@ -394,6 +394,18 @@ def load(description: Dict, index: Index) -> Manifest:
 def output(manifest: Manifest, res: Dict) -> Dict:
     """Convert a result into the v2 format"""
 
+    def collect_metadata(p: Pipeline) -> Dict[str, Any]:
+        data: Dict[str, Any] = {}
+        r = res.get(p.id, {})
+        for stage in r.get("stages", []):
+            md = stage.metadata
+            if not md:
+                continue
+            val = data.setdefault(stage.name, {})
+            val.update(md)
+
+        return data
+
     result: Dict[str, Any] = {}
 
     if not res["success"]:
@@ -424,16 +436,7 @@ def output(manifest: Manifest, res: Dict) -> Dict:
 
         # gather all the metadata
         for p in manifest.pipelines.values():
-            data: Dict[str, Any] = {}
-            r = res.get(p.id, {})
-            for stage in r.get("stages", []):
-                md = stage.metadata
-                if not md:
-                    continue
-                name = stage.name
-                val = data.setdefault(name, {})
-                val.update(md)
-
+            data: Dict[str, Any] = collect_metadata(p)
             if data:
                 result["metadata"][p.name] = data