Automatically append ellipsis (`...`) to metavars when `multiple=True` in options

When building a command-line interface with click.option(..., multiple=True), the auto-generated usage string does not visually signal to the user that the option can be repeated. For instance, an option --foo=FOO looks identical whether it accepts single or multiple arguments. This is different from the common convention for multi-value items in shell interfaces often suggests adding an ellipsis like --foo=FOO...

Expected behavior if option "foo" has multiple=True:

Usage: script.py [OPTIONS]

Options:
  --foo TEXT...  A list of foo strings.
  --help         Show this message and exit.

Trace Click's auto-generated option usage string for an option configured with multiple=True, comparing it with the single-value case. Add regression coverage for the expected --foo TEXT... output and verify that ordinary options remain unchanged.