CLI - play verb metavar (#1906)

better show `--clock [Hz]` than `--clock [CLOCK]`

Signed-off-by: Patrick Roncagliolo <[email protected]>

ros2bag/ros2bag/verb/play.py

@@ -89,7 +89,7 @@ def add_arguments(self, parser, cli_name):  # noqa: D102
                  'See storage plugin documentation for the format of this file.')
         clock_args_group = parser.add_mutually_exclusive_group()
         clock_args_group.add_argument(
-            '--clock', type=positive_float, nargs='?', const=40, default=0,
+            '--clock', type=positive_float, metavar='Hz', nargs='?', const=40, default=0,
             help='Publish to /clock at a specific frequency in Hz, to act as a ROS Time Source. '
                  'Value must be positive. Defaults to not publishing.'
                  'If specified, /clock topic in the bag file is excluded to publish.')