Don't require metric argument in cli tool

Author: cwasicki

src/frequenz/client/reporting/__main__.py

@@ -32,7 +32,8 @@ def main() -> None:
         nargs="+",
         choices=[e.name for e in Metric],
         help="List of metrics to process",
-        required=True,
+        required=False,
+        default=[],
     )
     parser.add_argument(
         "--states",