add prometheus tutorial (openyurtio#534)

* add prometheus tutorial
Congrool · Oct 30, 2021 · 4b3fa15 · 4b3fa15
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diff --git a/docs/img/prometheus.png b/docs/img/prometheus.png
diff --git a/docs/tutorial/prometheus.md b/docs/tutorial/prometheus.md
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+# prometheus
+
+this tutorial shows how to setup  prometheus to collects node metrics on the  edge through yurt-tunnel with DNS mode.Yurt-tunnel maintains a ConfigMap which will resolve edge nodeName to the address of  Yurt-tunnel server,and It will proxy traffic from cloud to edge.
+![system-architecture](../img/prometheus.png)
+## Local up OpenYurt cluster
+
+if you don't have the OpenYurt cluster, you can run the command [`local_up_openyurt.sh`](../../hack/local_up_openyurt.sh) to quickly set  up the OpenYurt cluster. you can reference this [turorial](./yurt-e2e-test.md) to figure out more details.
+
+in the OpenYurt work path,you can run the command as follows:
+
+```bash
+bash hack/local_up_openyurt.sh
+```
+
+If everything goes right, we will have a OpenYurt cluster running:
+
+```bash
+$ kubectl get node -o wide
+NAME                              STATUS   ROLES                  AGE     VERSION
+openyurt-e2e-test-control-plane   Ready    control-plane,master   4m15s   v1.20.7
+openyurt-e2e-test-worker          Ready    <none>                 3m42s   v1.20.7
+```
+
+## Install prometheus
+
+here we use [kube-prometheus-stack](https://github.com/prometheus-community/helm-charts/tree/main/charts/kube-prometheus-stack) to quickly install prometheus.
+
+```bash
+helm repo add prometheus-community https://prometheus-community.github.io/helm-charts
+helm repo update
+helm install prom prometheus-community/kube-prometheus-stack
+```
+
+## Modify the coreDNS config
+
+### 1.modify the deployment of coreDNS to mount the ConfigMap `yurt-tunnel-nodes`
+
+```bash
+kubectl patch deployment coredns -n kube-system  -p '{"spec": {"template": {"spec": {"volumes": [{"configMap":{"name":"yurt-tunnel-nodes"},"name": "edge"}]}}}}'
+kubectl patch deployment coredns -n kube-system   -p '{"spec": { "template": { "spec": { "containers": [{"name":"coredns","volumeMounts": [{"mountPath": "/etc/edge", "name": "edge", "readOnly": true }]}]}}}}'
+```
+
+### 2.modify the ConfigMap of coreDNS
+
+Use the [hosts](https://coredns.io/plugins/hosts/) plugin of coreDNS to add the dns records  into coreDNS.
+
+```bash
+$ kubectl edit configmap coredns -n kube-system
+...........
+ Corefile: |
+    .:53 {
+        errors
+        health {
+           lameduck 5s
+        }
+        ready
+        hosts /etc/edge/tunnel-nodes {    # add hosts plugin
+            reload 300ms
+            fallthrough
+        }
+        kubernetes cluster.local in-addr.arpa ip6.arpa {
+           pods insecure
+           fallthrough in-addr.arpa ip6.arpa
+           ttl 30
+        }
+        prometheus :9153
+        forward . /etc/resolv.conf {
+           max_concurrent 1000
+        }
+        cache 30
+        loop
+        reload
+        loadbalance
+    }
+```
+
+After this we should restart the coreDNS
+
+```bash
+kubectl patch deployment coredns -n kube-system -p '{"spec":{"template":{"spec":{"containers":[{"name":"coredns","env":[{"name":"RESTART","value":"'$(date +%s)'"}]}]}}}}'
+```
+
+## Modify the target of prometheus
+
+The prometheus targets  endpoint for kubernetes use host ip as default, we need to replace the host ip with hostname,so that we can fetch the metrics through the yurt-tunnel.
+
+- You can find more about relabel rule in the [prometheus docs](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#relabel_config).
+
+- And for different Kubernetes Labels you can take a look at this  [link](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#kubernetes_sd_config).
+
+### For kubelet
+
+ we need to modify the ServiceMonitor  `prom-kube-prometheus-stack-kubelet`
+
+```bash
+kubectl edit  servicemonitor  prom-kube-prometheus-stack-kubelet
+```
+
+here we use `__meta_kubernetes_endpoint_address_target_name` label to replace the host ip.
+
+```yaml
+spec:
+  endpoint:
+    ..........
+    relabelings:
+    - action: replace  # add relabel rule
+      regex: (.*);.*:(.*)
+      replacement: $1:$2
+      sourceLabels:
+      - __meta_kubernetes_endpoint_address_target_name
+      - __address__
+      targetLabel: __address__
+    ..........
+```
+
+### For others
+
+By default `Yurt-tunnel` only forwards two ports ,10250 and 10255,if we what to support more ports, we need to modfiy the ConfigMap`yurt-tunnel-server-cfg`,here we take`node-exporter` as an example. we need to add https port`9100`,so modify the `https-proxy-ports`.Similarly, if you want to  add http port, modify the `http-proxy-ports`.
+
+```bash
+kubectl patch configmap yurt-tunnel-server-cfg  -n kube-system  -p '{"data": {"https-proxy-ports":"9100"}}'
+```
+
+Like before, we modify the `ServiceMonitor` of node-exporter.
+
+```bash
+kubectl edit servicemonitor  prom-kube-prometheus-stack-node-exporter
+```
+
+here we use `__meta_kubernetes_pod_node_name` label to replace the host ip.
+
+```yaml
+spec:
+ endpoint:
+   ......
+   relabelings:
+    - action: replace #add relabel rule
+      regex: (.*);.*:(.*)
+      replacement: $1:$2
+      sourceLabels:
+      - __meta_kubernetes_pod_node_name
+      - __address__
+      targetLabel: __address__
+    ........
+```
+
+## Reference
+
+[Openyurt Yurt-Tunnel DNS模式实践](https://juejin.cn/post/7006898548415414279)