integrate @efiop 's suggestions

dvc/command/metrics.py

@@ -16,9 +16,9 @@ def show_metrics(metrics, all_branches=False, all_tags=False):
         metrics (list): Where each element is either a `list`
             if an xpath was specified, otherwise a `str`
     """
-    # When `metrics` contains a `_missing` key, it means that some files
+    # When `metrics` contains a `None` key, it means that some files
     # specified as `targets` in `repo.metrics.show` didn't contain any metrics.
-    missing = metrics.pop("_missing", None)
+    missing = metrics.pop(None, None)
 
     for branch, val in metrics.items():
         if all_branches or all_tags:

dvc/repo/metrics/show.py

@@ -276,12 +276,14 @@ def show(
     for branch in repo.brancher(all_branches=all_branches, all_tags=all_tags):
         metrics = {}
 
-        for p in targets:
-            entries = _collect_metrics(repo, p, recursive, typ, xpath, branch)
+        for target in targets:
+            entries = _collect_metrics(
+                repo, target, recursive, typ, xpath, branch
+            )
             metric = _read_metrics(repo, entries, branch)
 
             if metric:
-                found.add(p)
+                found.add(target)
                 metrics.update(metric)
 
         if metrics:
@@ -293,6 +295,6 @@ def show(
     missing = set(targets) - found
 
     if missing:
-        res["_missing"] = missing
+        res[None] = missing
 
     return res