subparsers is an example of choices displaying as the metavar.  From the documentation example:

usage: PROG [-h] [--foo] {a,b} ...

positional arguments:
  {a,b}   sub-command help

-----------------

To the main parser, the 'subparser' is a positional argument, with an optional 'dest' parameter (default is SUPPRESS), and {a,b} are the choices derived from the define subparser names.

The 'title' and 'description' parameters are used to create an Argument_group.

So any attempt to change how the metavar is created from choices has the potential of changing the subparser display.

That does suggest another formatting option - put your positional argument (with choices) in a custom Argument_Group.

In [658]: p=argparse.ArgumentParser()
In [659]: g=p.add_argument_group(title='lengths')
In [660]: g.add_argument('length',choices=[1,2,3]);
In [661]: p.print_help()
usage: ipython3 [-h] {1,2,3}

optional arguments:
  -h, --help  show this help message and exit

lengths:
  {1,2,3}