feature: add datadog metrics provider

[mathetake]

resolves #284

---

this PR adds an implmentation of metrics provider interface for datadog

references

• authentication validation endpoint: https://docs.datadoghq.com/api/?lang=bash#authentication
• metrics endpoint: https://docs.datadoghq.com/api/?lang=bash#query-timeseries-points

related issues

• Canary analysis metrics templating Canary analysis metrics templating #418
• Support other metrics sources InfluxDB metrics provider #241

TODOs

• test functions against an actual dd account
• e2e test against an actual dd account

example yamls

MetricTemplate

apiVersion: flagger.app/v1alpha1
kind: MetricTemplate
metadata:
  name: status-404-rate-on-datadog
spec:
  provider:
    type: datadog
    secretRef:
      name: datadog
    address: https://api.datadoghq.com
  query: |
    100 - (
      sum:istio.mesh.request.count{reporter:destination,destination_workload_namespace:{{ namespace }},destination_workload:{{ target }},!response_code:404}.as_count()
      / sum:istio.mesh.request.count{reporter:destination,destination_workload_namespace:{{ namespace }},destination_workload:{{ target }}}.as_count()
    ) * 100

Secret

apiVersion: v1
data:
  datadog_api_key: your-datadog-api-key
  datadog_application_key: your-datadog-application-key
kind: Secret
metadata:
  name: datadog

Canary

apiVersion: flagger.app/v1alpha3
kind: Canary
metadata:
  name: hello
  namespace: default
spec:
  canaryAnalysis:
    interval: 5s
    maxWeight: 50
    metrics:
      - interval: 30s
        name: datadog
        threshold: 99
        templateRef:
          name: status-404-rate-on-datadog
    stepWeight: 10
    threshold: 3
  targetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: hello
....

[stefanprodan]

LGTM

Thanks @mathetake