bug: tuple metavar not correctly displayed

```python
    test_selection = parser.add_argument_group("Test selection", "What should the script test")
    mutex_group.add_argument(
        "--test", "-t", metavar=("A", "B"), type=int, nargs=2, help="Select a specific A and B to run"
    )
```

will display under "Test selection":

```
--test A, -t A- Select a specific A and B to run (default: None)
```