Allow replacing commands with metavar when using parserOptionGroup

First of all, thank you for optparse-applicative! I've been using it for many years and it's one of the reasons I come back to Haskell every now and then.

Recently parserOptionGroup was added which allows grouping options. When used alone though, the options are shown twice. In niv, which has a few options (simplified below), it eats up a lot of real estate:

Usage: <interactive> update [PACKAGE] 
                            [
                              (--attribute KEY=VAL | 
                                --string-attribute KEY=VAL | --owner ARG | 
                                --repo ARG | --branch ARG | --rev ARG)]

  Update dependencies

ATTRIBUTES
  --attribute KEY=VAL      ...
  --string-attribute KEY=VAL
                           ...
  --owner ARG              ...
  --repo ARG               ...
  --branch ARG             ...
  --rev ARG                ...

Available options:
  -h,--help                Show this help text

I've come up with a mixture of a dummy option and hidden modifiers which show a metavar instead, which I think makes the output easier to understand:

Usage: <interactive> update [PACKAGE] [ATTRIBUTES]

  Update dependencies

ATTRIBUTES
  --attribute KEY=VAL      ...
  --string-attribute KEY=VAL
                           ...
  --owner ARG              ...
  --repo ARG               ...
  --branch ARG             ...
  --rev ARG                ...

Available options:
  -h,--help                Show this help text

The implementation looks like this:

parsePackageSpec' :: Opts.Parser ParsedPackageSpec
parsePackageSpec' = groupOptions "ATTRIBUTES" $ ParsedPackageSpec <$> Opts.some attribute
    where
        groupOptions :: String -> Opts.Parser a -> Opts.Parser a
        groupOptions mv x = Opts.option empty (Opts.metavar mv) <|> Opts.parserOptionGroup mv x
        
        attribute = Opts.strOption ... Opts.hidden

        ...

Am I missing something obvious with parserOptionGroup? Is there a simpler way to achieve this?

Start by reading the existing parserOptionGroup implementation and pull request #486, then compare the duplicated usage output with the proposed metavar workaround in this issue. Determine how grouped options can display a single metavar without the dummy option or hidden modifiers; done means the concise usage output is preserved while the ATTRIBUTES option descriptions remain visible.