def __init__(self, metric1, operator=None, metric2=None):
if not operator and metric2:<EOL><INDENT>raise ValueError("<STR_LIT>"<EOL>"<STR_LIT>")<EOL><DEDENT>self.metric1 = metric1<EOL>self.operator = operator<EOL>self.metric2 = metric2<EOL>
Create a CompoundMetricExpression.

        :param metric1: The metric on the left side of the operator
        :type metric1: `MetricExpression` or `CompoundMetricExpression`
        :param operator: The operator to use on metric1 and metric2
        :type operator: `str`
        :param metric2: The metric on the right side of the operator
        :type metric2: `MetricExpression` or `CompoundMetricExpression`

        Note that CompoundMetricExpression does respect standard order of
        operations (*/+-).

        >>> e = EventExpression('request')
        >>> m = MetricExpression('sum', e)
        >>> print(m + m)
        (sum(request) + sum(request))
        >>> print(m + m - m)
        ((sum(request) + sum(request)) - sum(request))
        >>> print(m + m * 2)
        (sum(request) + (sum(request) * 2))
        >>> print(m + m * m / m)
        (sum(request) + ((sum(request) * sum(request)) / sum(request)))