add monitoring dev guides

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

docs/en/guides/kubesphere-monitoring-development-guide.md

@@ -0,0 +1,49 @@
+# Monitoring
+
+This documentation contains backend development guides for interaction with Prometheus. The monitoring backend provides the capabilities of:
+
+ - Metrics query
+ - Metrics sorting
+ - Multi-tenant isolation
+
+## File Tree
+
+The listing below covers all folders related to the monitoring backend.
+
+```
+/pkg
+  ├─api
+  │  └─monitoring         # declares structs for api responses
+  │     └─v1alpha2
+  ├─apiserver             # implements handler for http requests
+  │  └─monitoring
+  ├─kapis                 # registers APIs and routing
+  │  └─monitoring
+  │     ├─install
+  │     └─v1alpha2
+  ├─models
+  │  └─metrics
+  │     ├─constants.go
+  │     ├─metrics.go       # proxies prometheus metrics query
+  │     ├─metrics_rules.go # promql expressions for builtin metrics
+  │     ├─namespaces.go    # appends metric info to namespace resource request
+  │     ├─types.go
+  │     └─util.go          # metrics sorting
+  └─simple
+     ├─factory.go          # factory functions for prometheus client options
+     └─client
+        └─prometheus
+           ├─options.go    # prometheus client options
+           └─prometheus.go # prometheus client code
+```
+
+## API Design
+
+To support multi-tenant isolation, the monitoring backend proxies Prometheus query requests. KubeSphere's APIs have the format like below:
+
+```
+GET /namespaces/{namespace}/pods
+GET /namespaces/{namespace}/pods/{pod}
+```
+
+KubeSphere API gateway will decode the URL and conduct authorization. A person who doesn't belong to a namespace will be rejected to make a request. Besides, note that the two examples above have slightly different meanings. The first is to retrieve all pod-level metrics in the namespace, while the latter is for a specific pod.