Support CloudWatch Alarms with metric math in alarm handler

[farski]

{'Period': 300, 'EvaluationPeriods': 1, 'ComparisonOperator': 'LessThanThreshold', 'Threshold': 10.0, 'TreatMissingData': '- TreatMissingData: missing', 'EvaluateLowSampleCountPercentile': '', 'Metrics': [{'Expression': 'MAX(METRICS())', 'Id': 'e1', 'Label': 'MAX(Evictions)', 'ReturnData': True}, {'Id': 'm1', 'MetricStat': {'Metric': {'Dimensions': [{'value': 'dovetail-prod-0001-001', 'name': 'CacheClusterId'}], 'MetricName': 'Evictions', 'Namespace': 'AWS/ElastiCache'}, 'Period': 300, 'Stat': 'Average'}, 'ReturnData': False}, {'Id': 'm2', 'MetricStat': {'Metric': {'Dimensions': [{'value': 'dovetail-prod-0001-002', 'name': 'CacheClusterId'}], 'MetricName': 'Evictions', 'Namespace': 'AWS/ElastiCache'}, 'Period': 300, 'Stat': 'Average'}, 'ReturnData': False}]}

Some values (like Namespace and Statistic) don't exist at the root when there's a metric math expression. Need to figure out how best to represent the that data as a single metric