add monitoring dev guides

Signed-off-by: huanggze <loganhuang@yunify.com>

docs/en/concepts-and-designs/kubesphere-monitoring.md

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+# KubeSphere Monitoring
+
+## Overview
+
+The KubeSphere monitoring system comprises many components that work together to achieve overall monitoring functionality. KubeSphere defaults to installing the monitoring module. It will deploy  or create a few components within the namespace `kubesphere-monitoring-system`:
+
+- Prometheus Operator: automates tasks for operating Prometheus instances.
+- Prometheus: scrapes metrics and provides monitoring service.
+- Kube-state-metrics: exposes metrics for kubernetes objects like deployment, pod, namespace, etc.
+- Node-exporter: exposes node metrics.
+
+Besides, the monitoring stack includes CRD resources:
+
+- ServiceMonitor: declaratively specifies how groups of services should be monitored.
+- Prometheus: defines a desired Prometheus deployment.
+- PrometheusRule: defines a desired Prometheus rule file.
+
+In KubeSphere, Prometheus will monitor the following services by default: 
+
+- kube-state-metrics
+- node-exporter
+- kubelet
+- s2i-operator
+- etcd
+- coredns
+- kube-apiserver
+- kube-scheduler
+- kube-controller-manager
+
+## Setup
+
+The [contrib/kube-prometheus](https://github.com/kubesphere/prometheus-operator/tree/ks-v0.27.0/contrib/kube-prometheus) folder provides manifests for setting up the monitoring stack. The [kubernetes-mixin/rules](https://github.com/kubesphere/kubernetes-mixin/blob/ks-v0.27.0/rules/rules.libsonnet) project provides recording rule templates. KubeSphere monitoring backend serves as Prometheus client performing metrics query.
+
+The diagram below presents the overall monitoring architecture:
+
+![](../../images/kubesphere-monitoring-architecture.png)