Incorrect help message on wrong flags for `metrics` and `plots`

[skshetry]

Bug Report

$ dvc plots diff --flags
ERROR: unrecognized arguments: --flag
usage: dvc diff [-h] [-q | -v] [--show-json] [--show-hash] [--show-md]
                [a_rev] [b_rev]
... (truncated)

$ dvc metrics diff --flags
ERROR: unrecognized arguments: --flags
usage: dvc diff [-h] [-q | -v] [--show-json] [--show-hash] [--show-md]
                [a_rev] [b_rev]
... (truncated)

$ dvc metrics show --flags
ERROR: unrecognized arguments: --flags
usage: dvc [-q | -v] [-h] [-V] COMMAND ...
... (truncated)