cli: lowercase metavar by default (#10227)

* cli: lowercase metavar by default

* add tests for help message

tests/unit/command/test_help.py

@@ -1,5 +1,7 @@
 import logging
+import re
 from argparse import SUPPRESS, ArgumentParser
+from itertools import takewhile
 from typing import Tuple
 
 import pytest
@@ -44,5 +46,19 @@ def test_help(caplog, capsys, command_tuples):
     assert not caplog.text
 
     out, err = capsys.readouterr()
+
+    # validate metavars are all in lowercase
+    usage = "\n".join(takewhile(lambda o: bool(o), out.splitlines()))
+
+    message = (
+        "metavars not lowercased, you are likely missing formatter_class=XXX "
+        "in the command, where XXX should be any of the classes from "
+        "`dvc.cli.formatter`, which automatically lowercases metavars.\n"
+        "\nExample:\n"
+        "\nfrom dvc.cli import formatter\n"
+        "\nparser.add_parser(..., formatter_class=formatter.TextHelpFormatter)\n"
+    )
+    assert not re.findall(r"\b[A-Z]{2,}\b", usage, re.MULTILINE), message
+
     assert not err
     assert out