pkg/metrics/providers: add AWS CloudWatch provider

[mathetake]

resolves #465

---

this PR implements metrics provider interface for AWS CloudWatch

TODOs

• e2e test against an actual AWS account
• make change to crd

related issue

#241

note

• users must give AWS IAM permission to flagger to perform cloudwatch:GetMetricData
• AWS's region is specified by provider.region field

example

MetricTemplate

apiVersion: flagger.app/v1alpha1
kind: MetricTemplate
metadata:
  name: cwtest
spec:
  provider:
    type: cloudwatch
    region: ap-northeast-1
  query: |
    [
        {
            "Id": "e1",
            "Expression": "m1 / m2",
            "Label": "ErrorRate"
        },
        {
            "Id": "m1",
            "MetricStat": {
                "Metric": {
                    "Namespace": "MyApp",
                    "MetricName": "ErrorCount",
                    "Dimensions": [
                        {
                            "Name": "appName",
                            "Value": "{{ name }}.{{ namespace }}"
                        }
                    ]
                },
                "Period": 60,
                "Stat": "Sum",
                "Unit": "Count"
            },
            "ReturnData": false
        },
        {
            "Id": "m2",
            "MetricStat": {
                "Metric": {
                    "Namespace": "MyApp",
                    "MetricName": "RequestCount",
                    "Dimensions": [
                        {
                            "Name": "appName",
                            "Value": "{{ name }}.{{ namespace }}"
                        }
                    ]
                },
                "Period": 60,
                "Stat": "Sum",
                "Unit": "Count"
            },
            "ReturnData": false
        }
    ]

Canary

apiVersion: flagger.app/v1alpha3
kind: Canary
metadata:
  name: prd-istio-gateway
  namespace: default
spec:
  canaryAnalysis:
    interval: 5s
    maxWeight: 50
    metrics:
      - interval: 30s
        name: cloudwatch
        threshold: 99
        thresholdRange:
          min: 0.4
          max: 0.9
        templateRef:
          name: cwtest
    stepWeight: 10
    threshold: 3
...

[stefanprodan]

Please add an "AWS CloudWatch" to the https://github.com/weaveworks/flagger/blob/master/docs/gitbook/usage/metrics.md

Thanks!

[stefanprodan]

LGTM

Thanks @mathetake 🏅