cmd ref: add --all to metrics diff

per #1266 (review)

content/docs/command-reference/metrics/diff.md

@@ -9,9 +9,8 @@ commits in the <abbr>DVC repository</abbr>, or between a commit and the
 ```usage
 usage: dvc metrics diff [-h] [-q | -v]
                         [--targets [<path> [<path> ...]]]
-                        [-t <type>] [-x <path>] [-R]
-                        [--show-json] [--show-md]
-                        [a_ref] [b_ref]
+                        [-t <type>] [-x <path>] [-R] [--all]
+                        [--show-json] [--show-md] [a_ref] [b_ref]
 
 positional arguments:
   a_rev                 Old Git commit to compare (defaults to HEAD)
@@ -61,6 +60,8 @@ They're calculated between two commits (hash, branch, tag, or any
   path is allowed. It will override `xpath` defined in the corresponding
   DVC-file. See `dvc metrics show` for more details.
 
+- `--all` - list all metrics, even those without differences to compare.
+
 - `--show-json` - prints the command's output in easily parsable JSON format,
   instead of a human-readable table.