feat(cw): handle alerts with multiple metrics

[rfvermut]

i.e. anomaly detection alerts

sample event:

{
  "Records": [
    {
      "EventSource": "aws:sns",
      "EventVersion": "1.0",
      "EventSubscriptionArn": "arn:aws:sns:us-east-1:AAAAXXXXX:data--alerts:34a7b0cb-3f8c-4471-aea2-2bbc7c4eacc9",
      "Sns": {
        "Type": "Notification",
        "MessageId": "6b034f0c-b465-58da-a999-179029ec5364",
        "TopicArn": "arn:aws:sns:us-east-1:AAAAXXXXX:data--alerts",
        "Subject": "OK: \"text-CryptosFunctionInvocationsAlarm-G6D30JIL9N7T\" in US East (N. Virginia)",
        "Message": "{\"AlarmName\":\"text-CryptosFunctionInvocationsAlarm-G6D30JIL9N7T\",\"AlarmDescription\":null,\"AWSAccountId\":\"AAAAXXXXX\",\"NewStateValue\":\"OK\",\"NewStateReason\":\"Thresholds Crossed: 1 out of the last 5 datapoints [1.0 (09/02/21 14:23:00)] was not less than the lower thresholds [0.96] or not greater than the upper thresholds [1.04] (minimum 3 datapoints for ALARM -> OK transition).\",\"StateChangeTime\":\"2021-02-09T14:25:13.236+0000\",\"Region\":\"US East (N. Virginia)\",\"AlarmArn\":\"arn:aws:cloudwatch:us-east-1:AAAAXXXXX:alarm:text-CryptosFunctionInvocationsAlarm-G6D30JIL9N7T\",\"OldStateValue\":\"INSUFFICIENT_DATA\",\"Trigger\":{\"Period\":60,\"EvaluationPeriods\":5,\"ComparisonOperator\":\"LessThanLowerOrGreaterThanUpperThreshold\",\"ThresholdMetricId\":\"ad1\",\"TreatMissingData\":\"- TreatMissingData:                    missing\",\"EvaluateLowSampleCountPercentile\":\"\",\"Metrics\":[{\"Id\":\"m1\",\"MetricStat\":{\"Metric\":{\"Dimensions\":[{\"value\":\"text-cryptos\",\"name\":\"FunctionName\"}],\"MetricName\":\"Invocations\",\"Namespace\":\"AWS/Lambda\"},\"Period\":60,\"Stat\":\"Average\"},\"ReturnData\":true},{\"Expression\":\"ANOMALY_DETECTION_BAND(m1, 2)\",\"Id\":\"ad1\",\"Label\":\"Invocations (expected)\",\"ReturnData\":true}]}}",
        "Timestamp": "2021-02-09T14:25:13.277Z",
        "SignatureVersion": "1",
        "Signature": "phodF/+ksm24Zc85XbCxHo+HhIz1f0d71NV8bGhWT1KfBY4NKPj9K+18f+DxZO2U74TFs+ZrGEKZ8YMDAlptjkbcrMQ4N4R5RP4yk5oQDQxDG0abLJITWEsZqmZNkxa37mdNsY0phJ1F0wAZTB8IdxIehNzkBJqDMnSJeMaYYnDmTXo53vIb5HLAUamoxak/rvpUX1FyGCnWgwc+xLU47X9uHjKvriGnQSQSQLu5+Tc0fP8I7NS2zDIK1XZKs8SZeclkx1XPE3yPS5W9R335A1XqKG+QMdCObJKZ5S6umOFxaEMuJQeSlrtA9DgEfDO3/VUUwOHBIUll9z0mgQ8fmw==",
        "SigningCertUrl": "https://sns.us-east-1.amazonaws.com/SimpleNotificationService-010a507c1833636cd94bdb98bd93083a.pem",
        "UnsubscribeUrl": "https://sns.us-east-1.amazonaws.com/?Action=Unsubscribe&SubscriptionArn=arn:aws:sns:us-east-1:AAAAXXXXX:data--alerts:34a7b0cb-3f8c-4471-aea2-2bbc7c4eacc9",
        "MessageAttributes": {}
      }
    }
  ]
}