Update backend-fetcher.md (#5972)

CHANGES.md

@@ -28,7 +28,7 @@ Release Notes.
 * Fix trace instance selector bug.
 
 #### Documentation
-
+* Update the documents of backend fetcher and self observability about the latest configurations.
 
 All issues and pull requests are [here](https://github.com/apache/skywalking/milestone/68?closed=1)
 

docs/en/setup/backend/backend-fetcher.md

@@ -3,11 +3,14 @@ Fetcher is a concept in SkyWalking backend. It uses pulling mode rather than [re
 read the data from the target systems. This mode is typically in some metrics SDKs, such as Prometheus.
 
 ## Prometheus Fetcher
+Suppose you want to enable some `metric-custom.yaml` files stored at `fetcher-prom-rules`, append its name to `enabledRules` of 
+ `promethues-fetcher` as below:
+ 
 ```yaml
 prometheus-fetcher:
   selector: ${SW_PROMETHEUS_FETCHER:default}
   default:
-    active: ${SW_PROMETHEUS_FETCHER_ACTIVE:false}
+    enabledRules: ${SW_PROMETHEUS_FETCHER_ENABLED_RULES:"self,metric-custom"}
 ```
 
 ### Configuration file

docs/en/setup/backend/backend-telemetry.md

@@ -74,7 +74,7 @@ telemetry:
 prometheus-fetcher:
   selector: ${SW_PROMETHEUS_FETCHER:default}
   default:
-    active: ${SW_PROMETHEUS_FETCHER_ACTIVE:true}
+    enabledRules: ${SW_PROMETHEUS_FETCHER_ENABLED_RULES:"self"}
 ``` 
 
 3. Make sure `config/fetcher-prom-rules/self.yaml` exists.