docs: Add prometheus-operator tutorial

docs/gitbook/tutorials/prometheus-operator.md

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+# Flagger with Prometheus Operator
+
+This guide will show you how to use Flagger and Prometheus Operator.  
+This guide will handle only Blue/Green Deployment with podinfo application
+
+## Prerequisites
+
+Flagger and Prometheus Operator requires a Kubernetes cluster **v1.11** or newer
+
+Install Prometheus-Operator with Helm v3:
+
+```bash
+helm repo add stable https://kubernetes-charts.storage.googleapis.com
+helm repo update
+kubectl create ns monitoring
+helm upgrade -i prometheus stable/prometheus-operator \
+--namespace monitoring \
+--set prometheus.prometheusSpec.serviceMonitorSelectorNilUsesHelmValues=false \
+--set fullnameOverride=prometheus
+```
+
+The `prometheus.prometheusSpec.serviceMonitorSelectorNilUsesHelmValues=false` option allows Prometheus-Operator to watch serviceMonitor outside of his namespace.
+
+You can also set `prometheus.service.type=nodePort` if you want to have access the Prometheus UI
+
+Install Flagger with Helm v3:
+
+```bash
+helm repo add flagger https://flagger.app
+helm repo update
+kubectl create ns flagger
+helm upgrade -i flagger flagger/flagger \
+--namespace flagger \
+--set metricsServer=http://prometheus-prometheus.monitoring:9090 \
+--set meshProvider=kubernetes
+```
+
+The `meshProvider` option can be changed to your value, if you want to do something else than Blue/Green Deployment
+
+Install Flagger Loadtester with Helm v3:
+
+```bash
+helm repo add flagger https://flagger.app
+helm repo update
+kubectl create ns flagger
+helm upgrade -i loadtester flagger/loadtester \
+--namespace flagger
+```
+
+Install podinfo with Helm v3:
+
+```bash
+helm repo add sp https://stefanprodan.github.io/podinfo
+helm repo update
+kubectl create ns test
+helm upgrade -i podinfo sp/podinfo \
+--namespace test
+```
+
+## Setting ServiceMonitor
+
+Prometheus Operator is using mostly serviceMonitor instead of annotations.  
+In order to catch metrics for primary and canary service, you will need to create 2 serviceMonitors :
+
+```yaml
+apiVersion: monitoring.coreos.com/v1
+kind: ServiceMonitor
+metadata:
+  name: podinfo
+  namespace: test
+spec:
+  endpoints:
+  - path: /metrics
+    port: http
+    interval: 15s
+  selector:
+    matchLabels:
+      app: podinfo
+```
+
+```yaml
+apiVersion: monitoring.coreos.com/v1
+kind: ServiceMonitor
+metadata:
+  name: podinfo-canary
+  namespace: test
+spec:
+  endpoints:
+  - path: /metrics
+    port: http
+    interval: 15s
+  selector:
+    matchLabels:
+      app: podinfo-canary
+```
+
+We are setting `interval: 15s` to have a more aggressive scraping
+If you do not define it, you must to use a longer interval in the Canary object
+
+## Setting Custom metrics
+
+Prometheus Operator is relabeling for every serviceMonitor, you can create custom metrics to you own filter.
+
+```yaml
+apiVersion: flagger.app/v1beta1
+kind: MetricTemplate
+metadata:
+  name: request-success-rate
+  namespace: test
+spec:
+  provider:
+    address: http://prometheus-prometheus.monitoring:9090
+    type: prometheus
+  query: |
+    rate(
+      http_requests_total{
+        namespace="{{ namespace }}",
+        job="{{ target }}-canary",
+        status!~"5.*"
+      }[{{ interval }}]) 
+    / 
+    rate(
+      http_requests_total{
+        namespace="{{ namespace }}",
+        job="{{ target }}-canary"
+      }[{{ interval }}]
+    ) * 100
+```
+
+You can also use `pod="{{ target }}-[0-9a-zA-Z]+(-[0-9a-zA-Z]+)"` instead of `job={{ target }}-canary`, if you want.
+
+## Creating Canary
+
+```yaml
+apiVersion: flagger.app/v1beta1
+kind: Canary
+metadata:
+  name: podinfo
+  namespace: test
+spec:
+  provider: kubernetes
+  targetRef:
+    apiVersion: apps/v1
+    kind: Deployment
+    name: podinfo
+  progressDeadlineSeconds: 60
+  service:
+    port: 9898
+    portDiscovery: true
+  analysis:
+    interval: 30s
+    iterations: 10
+    threshold: 2
+    metrics:
+    - name: http-success-rate
+      templateRef:
+        name: request-success-rate
+        namespace: test
+      thresholdRange:
+        min: 99
+      interval: 1m
+    webhooks:
+      - name: smoke-test
+        type: pre-rollout
+        url: "http://loadtester.flagger/"
+        timeout: 15s
+        metadata:
+          type: bash
+          cmd: "curl -sd 'anon' http://podinfo-canary.test:9898/token | grep token"
+      - name: load-test
+        type: rollout
+        url: "http://loadtester.flagger/"
+        timeout: 5s
+        metadata:
+          type: cmd
+          cmd: "hey -z 1m -q 10 -c 2 http://podinfo-canary.test:9898"
+```
+
+## Test the canary
+
+Execute `kubectl -n test set image deployment/podinfo podinfo=stefanprodan/podinfo:3.1.0` to see if everything works
+
+
+