'Return a single metadata item.'

@extensions.expected_errors(404)
def show(self, req, server_id, id):

    context = req.environ['nova.context']
    context.can((sm_policies.POLICY_ROOT % 'show'))
    data = self._get_metadata(context, server_id)
    try:
        return {'meta': {id: data[id]}}
    except KeyError:
        msg = _('Metadata    item    was    not    found')
        raise exc.HTTPNotFound(explanation=msg)
