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
+
+## Prerequisites
+
+Flagger requires a Kubernetes cluster **v1.11** or newer and NGINX ingress **0.24** 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 prometheus
+helm upgrade -i prometheus stable/prometheus-operator \
+--namespace prometheus \
+--set prometheus.prometheusSpec.serviceMonitorSelectorNilUsesHelmValue=false
+```
+The `prometheus.prometheusSpec.serviceMonitorSelectorNilUsesHelmValue` allows Prometheus-Operator to watch serviceMonitor outside of his namespace.
+
+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 prometheus-prometheus-oper-prometheus.prometheus:9090 \
+--set meshProvider=kubernetes
+```
+
+The `meshProvider` option can be changed to your value
+
+## 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: service-monitor-primary
+  namespace: application
+spec:
+  endpoints:
+  - path: /metrics
+    port: http
+  selector:
+    matchLabels:
+      app: application
+```
+
+```yaml
+apiVersion: monitoring.coreos.com/v1
+kind: ServiceMonitor
+metadata:
+  name: service-monitor-canary
+  namespace: application
+spec:
+  endpoints:
+  - path: /metrics
+    port: http
+  selector:
+    matchLabels:
+      app: application-canary
+```
+
+## Setting Custom metrics
+
+Prometheus Operator is relabeling for every serviceMonitor, you can create custom metrics
+
+```yaml
+apiVersion: flagger.app/v1beta1
+kind: MetricTemplate
+metadata:
+  name: your-custom-metrics
+  namespace: application
+spec:
+  provider:
+    address: http://prometheus-prometheus-oper-prometheus.prometheus:9090
+    type: prometheus
+  query: |
+    100 - sum(
+          rate(
+            http_server_requests_seconds_max{
+              namespace="{{namespace }}",
+            job="{{ target }}-canary",
+            status!~\"5.*\"
+        }[{{ interval }}]
+        )
+      ) 
+      / 
+      sum(
+        rate(
+            http_server_requests_seconds_max{
+            namespace="{{ namespace }}",
+            job="{{ target }}-canary"
+        }[{{ interval }}]
+        )
+      ) 
+      * 100
+```
+
+You can use the job to filter instead of pod. You can tune it to your installation.
+
+## Creating Canary
+
+```yaml
+apiVersion: flagger.app/v1beta1
+kind: Canary
+metadata:
+  name: application
+  namespace: security-manager
+spec:
+  analysis:
+    interval: 30s
+    iterations: 10
+    threshold: 5
+    metrics:
+    - interval: 1m
+      name: http-requests-seconds
+      templateRef:
+        name: your-custom-metrics
+        namespace: application
+      thresholdRange:
+        max: 1
+    webhooks: []
+  progressDeadlineSeconds: 600
+  provider: kubernetes
+  service:
+    port: 8080
+    portDiscovery: true
+  targetRef:
+    apiVersion: v1
+    kind: Deployment
+    name: application
+```
+
+You can define the webhooks of your liking also
+
+