Add correction to error message formatter (_get_action_name)
Use _format_metavars to adapt the tuple metavar for both cases (help and error)
'|'.join  is one possibility (used in test_argparse)
alternatives include str(), [0], and a call to _format_args(), which formats the metavar as used in usage.
(this last alternative is attractive, but involves a bit of ugly programming).
Merge branch 'issue14074' into composite