add High Availability capability for Prometheus

• test Prometheus running with multiple replicas

• ensure thanos configuration works across Prometheus instances

• thanos can be pulled in currently with these values and helm command :

  objstoreConfig: |-
    type: s3
    config:
      bucket: thanos
      endpoint: {{ include "thanos.minio.fullname" . }}.monitoring.svc.cluster.local:9000
      access_key: minio
      secret_key: minio123
      insecure: true
  query:
    dnsDiscovery:
      sidecarsService: kube-prometheus-prometheus-thanos
      sidecarsNamespace: monitoring
  bucketweb:
    enabled: true
  compactor:
    enabled: true
  storegateway:
    enabled: true
  ruler:
    enabled: true
    alertmanagers:
      - http://kube-prometheus-alertmanager.monitoring.svc.cluster.local:9093
    config: |-
      groups:
        - name: "metamonitoring"
          rules:
            - alert: "PrometheusDown"
              expr: absent(up{prometheus="monitoring/kube-prometheus"})
  metrics:
    enabled: true
    serviceMonitor:
      enabled: true
  minio:
    enabled: true
    accessKey:
      password: "minio"
    secretKey:
      password: "minio123"
    defaultBuckets: "thanos"

    helm install thanos \
      --values thanos-values.yaml \
      --namespace monitoring \
      bitnami/thanos

• update HA section of arch doc : ~/bigbang/docs/understanding-bigbang/package-architecture/monitoring.md