Add options specific to metrics subcommand.

def _metrics_options(p):
    """ Add options specific to metrics subcommand. """
    _default_options(p, blacklist=['log-group', 'output-dir', 'cache', 'quiet'])

    p.add_argument(
        '--start', type=date_parse,
        help='Start date (requires --end, overrides --days)')
    p.add_argument(
        '--end', type=date_parse, help='End date')
    p.add_argument(
        '--days', type=int, default=14,
        help='Number of days of history to consider (default: %(default)i)')
    p.add_argument('--period', type=int, default=60 * 24 * 24)